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Biology and Conservation Needs of the Short-tailed Hawk in Florida

FINAL REPORT Kenneth D. Meyer

July 2005

Florida Fish and Wildlife Conservation Commission 620 South Meridian Street Tallahassee, FL 32399-1600


Biology and Conservation Needs of the Short-tailed Hawk in Florida

Kenneth D. Meyer

Avian Research and Conservation Institute 411 N.E. 7th Street, Gainesville, FL 32601

Submitted as final report for Florida Fish and Wildlife Conservation Commission Project NG99-102 July 2005


This report is the result of a project supported by the Florida Fish and Wildlife Conservation Commission’s Nongame Wildlife Trust Fund. It has been reviewed for clarity, style, and typographical errors, but has not received peer review. Any opinions or recommendations in this report are those of the authors and do not represent policy of the Commission.

Suggested citation: Meyer, K. D. 2005. Biology and conservation of the short-tailed hawk in Florida. Final report. Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA.


Biology and Conservation Needs of the Short-tailed Hawk in Florida

Kenneth D. Meyer Avian Research and Conservation Institute, 411 N.E. 7th Street, Gainesville, FL 32601

Abstract: The short-tailed hawk (Buteo brachyurus) is one of Florida’s rarest and least-studied vertebrates. Although the range of this small buteo extends from Mexico southward through much of South America, its U.S. distribution is limited to Florida. Very small population size combined with low productivity and lack of information on the population’s distribution, habitat needs, status, and trends make the short-tailed hawk a high priority for research and survey efforts. This report covers work from October 1999 to December 2001, or 2 full wintering and breeding seasons. Objectives of the study were to solicit sighting reports, use hawks radio tagged during winter and ground-searches of historic and newly reported nesting areas to find nests, monitor nests to determine nesting success and productivity, collect nest-site vegetation data, radio tag young and adults near their nests, determine breeding-season foraging habitats and range sizes of radio-tagged adults, evaluate first-year survival and likely causes of death for radio-tagged young, radio tag wintering short-tailed hawks in southern Florida, and determine winter foraging habitats and range sizes of radio-tagged hawks. We used 38 VHF radio-tagged hawks, captured mainly during winter, and 1 bird carrying a satellite transmitter to address the movement, habitat, activity range, and survival objectives. A major underlying objective was to develop feasible field methods for finding nests and capturing, radio tagging, and tracking hawks. We conclude from our studies that the Florida population most likely includes <200 pairs attempting to nest in any year. This very small population size makes Florida’s short-tailed hawks highly vulnerable. Sex ratio should be examined further due to the suggestion that females may be underrepresented among nestlings. There is no question that the short-tailed hawk’s reproductive performance ranks at the very lowest end of the scale in comparison with similar species. Low nesting success in our studies resulted almost entirely from nest failures during the egg stage. An encouraging conclusion is that fledging success for young that hatch is very good, and first-year survival appears to be high. In all cases studied, short-tailed hawks in Florida nested in dense, mature stands of wetland forest. Nests that were found by radio telemetry, a method unbiased by physical access and observability of the nest stand, were in the interior of large stands of dense, mature, wetland forest, areas that are becoming increasingly rare on Florida’s privately owned lands. Development and implementation of a long-term monitoring plan for short-tailed hawks should be given high priority. Federal listing of short-tailed hawks is rarely discussed, perhaps because the population-trend requirement for securing Endangered or Threatened status has been so difficult to pursue for this species. This should serve as additional justification for implementing a sound monitoring plan. We should remember, however, that in the case of the very small and minimally productive Florida population of short-tailed hawks, it may be impossible or prohibitively expensive to reverse a declining trend even if action is taken as soon as the decline is detected.

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ACKNOWLEDGMENTS I thank the following: National Park Service, Everglades National Park, for granting permission to trap, mark, and study short-tailed hawks; Sonny Bass of Everglades National Park for arranging a helicopter flight to locate the nest on Cape Sable; Ken Tracey for sharing his knowledge of the nest in Pasco County; Florida Keys Audubon Society for information and assistance in Key West; Casey Lott for his skillful help with trapping; Karl Miller for help with statistical analyses at earlier stages of the project; Mike Avery and USDAâ&#x20AC;&#x2122;s Denver Wildlife Research Laboratory for providing lure birds; Frank Ogborn and Jim Hendrix for competent and safe piloting services; and The Timber Company, Brian Alford of Alford Timber Company, The Nature Conservancy, KSA Rodeo Ranch, Caber Corp., and Lake Arbuckle State Forest for access to their land. Funding for this project came mainly from the Nongame Wildlife Program (Contracted Projects Program) of Florida Fish and Wildlife Conservation Commission (FWC); I thank FWCâ&#x20AC;&#x2122;s Stuart Cumberbatch and Michael Evans for their help in establishing and administering the contract, Cavell Kyser for her patient and careful editing, and Randy Kautz for providing the digitized vegetation and land use classification file used in our habitat analyses, as well as advice and other information. The project proposal did not call for any satellite telemetry. The transmitter, with a retail value of $2,900, was donated by Microwave Telemetry. Tracking time, provided by Service Argos, at an average cost of about $1,500 per transmitter per year, was purchased by Avian Research and Conservation Institute with funds from unrestricted contributions, including a donation from the Felburn Foundation. Finally, I am especially grateful to Gina Zimmerman, Tim Morris, Tim Dellinger, Rachel Ridall, Laura Barjaktarovic, and Bradley Richardson for their careful and enthusiastic field work under difficult conditions. Gina Zimmerman helped to produce the GIS vegetation and activity range analyses and assisted with completing the final report.

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TABLE OF CONTENTS ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Sighting Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Trapping and Radio Tagging During Winter . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Finding Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Monitoring Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Describing Nest-site Vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Trapping and Radio Tagging Adults and Young at Nests . . . . . . . . . . . . . . . . . . 9 Describing Summer Activity Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Describing Winter Activity Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Estimating Survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Trapping and Radio Tagging During Winter . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Finding Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Monitoring Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Nest-site Vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Trapping and Radio Tagging Adults and Young at Nests . . . . . . . . . . . . . . . . . 21 Summer Activity Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Winter Activity Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Sex and Color-morph Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Trapping and Radio Tagging During Winter . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Finding Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Monitoring Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Nest-site Vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Trapping and Radio Tagging Adults and Young at Nests . . . . . . . . . . . . . . . . . 39 Summer Activity Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Winter Activity Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Color Morph and Sex Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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IMPLICATIONS FOR MANAGEMENT AND CONSERVATION . . . . . . . . . 45 Population Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Low Nesting Success and Productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Nesting Habitat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Additional Management Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

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INTRODUCTION The short-tailed hawk (Buteo brachyurus) is one of Florida’s rarest and least-studied vertebrates. Although the range of this small buteo extends from Mexico southward through much of South America, its U.S. distribution is limited to Florida, over 800 km from any other population (Brown and Amadon 1968, Ogden 1988, Miller and Meyer 2002). Short-tailed hawks reside in Florida year-round, though they migrate seasonally within the state (Ogden 1974). They specialize on birds, unusual prey for buteos, and they exhibit 2 strikingly different plumage morphs. Effective and affordable survey methods for short-tailed hawks have not been developed, and no one has claimed to make an accurate estimate of Florida’s population. During the 1930s, Sprunt (1939) perceived a decline, which Ogden (1988) discounted as an effect of inconsistent observer effort. Certain important nesting areas have been lost to logging and development, most notably around Lake Istokpoga (Brandt 1924, Nicholson 1951), but Ogden (1988) noted that the recent statewide distribution is essentially the same as that described by early naturalists. Nonetheless, there has never been a means to detect changes in population size over time, except indirectly by noting abandonment of some traditional nest sites or by identifying activity at some previously unreported locations, the latter being as likely due to chance or increased search effort as to new occupancy. Millsap et al. (1996) reported that 43 breeding-season locations indicative of nesting have been documented since 1951. Ogden (1988, 46) stated that short-tailed hawks “may total fewer that 500 in Florida,” presumably including nonbreeders and young of the year. Millsap et al. (1989) concurred with this upper limit and suggested that “The number may be much smaller.” Based on Ogden’s (1988) estimate, the number of breeding pairs in any year probably would not exceed 150–200. Small population size combined with low productivity (Ogden 1974, Millsap et al. 1989) and lack of information on the population’s distribution, habitat needs, status, and trends make a clear case for considering the shorttailed hawk a high-priority species for research and survey efforts. Indeed, Florida Fish and Wildlife Conservation Commission (FWC), in assessing wildlife conservation priorities, ranked the short-tailed hawk eighteenth overall among Florida’s extant vertebrate species with regard to the state of knowledge and need for additional work (Millsap et al. 1990). The species was ranked first among vertebrates in need of increased research and population surveys. The short-tailed hawk’s Biological Score of 36 in FWC’s analysis was above the median score for species presently state listed as Endangered. Finally, FWC’s review ranked 4 habitats critical to short-tailed


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hawks (Millsap et al. 1989, 1996) among the highest of the state’s ecological communities in terms of biological importance and sensitivity. Short-tailed hawks were once thought to vacate southern Florida during the breeding season (Moore et al. 1953). It is now clear, however, that they do breed in southern Florida but become more concentrated there during winter (Ogden 1974, 1988; Stevenson and Anderson 1994; Millsap et al. 1996). Ogden (1974) described a true migration for the Florida population (i.e., predictable dates and a consistent pattern, not merely dispersion), although they apparently move singly or in small numbers (Ogden 1988). While the majority of Florida’s short-tailed hawks retreat south of Lake Okeechobee for the winter (Ogden 1974) and occupy a relatively small area (Ogden 1988, Millsap et al. 1989), some resident individuals are regularly reported in northern Florida (see Robertson and Woolfenden 1992). Some individuals wintering in southern Florida frequent suburbs and prey on exotic doves (Streptopelia decaocto and Zenaida asiatica) (Robertson and Woolfenden 1992). Most, however, concentrate in Everglades National Park, where they remain inconspicuous by occupying transitional zones between mangrove swamps and either tidal marsh, coastal prairie, or sawgrass sloughs, which they apparently use in disproportion to availability (Millsap et al. 1989). Factors that increase the short-tailed hawk’s vulnerability to extirpation include small population size, small U.S. distribution, loss of historic nest sites, concentrated winter distribution, and specialized diet (Millsap et al. 1996). These threatening factors are exacerbated by the short-tailed hawk’s low productivity (Ogden 1988, Millsap et al. 1989) and by our poor understanding of the species’ status, distribution, and habitat requirements. The short-tailed hawk’s vulnerability is clearly reflected in the FWC’s veryhigh-priority conservation status for the species, particularly with regard to the need for research and survey attention (Millsap et al. 1990). The short-tailed hawk’s wetland nest sites are, theoretically, protected by federal and state regulations, but permitted incremental losses of wetlands, particularly in cypress and mixed-species swamp forests on the fringes of lakes, creeks, and rivers, will continue to take nest sites. The species’ high fidelity to nest sites greatly increases their vulnerability to loss of critical habitat and makes it essential that managers identify and protect as many of these sites as possible. The fact that known nest sites lie within largely unaltered landscapes highlights the importance of intact native vegetation communities as foraging habitat for nesting short-tailed hawks (Millsap et al. 1996). These habitats are highly vulnerable to agricultural and residential development. While it is


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essential to protect and maintain use of historic nest sites, the much larger area required for supporting a pair of hawks and their developing young means that loss of foraging habitat is the most imposing threat to Florida’s population of short-tailed hawks (Millsap et al. 1996). What is known of the short-tailed hawk’s distribution in Florida indicates that suitably large expanses of public lands support an insufficient number of short-tailed hawk territories to maintain the population. It is imperative, therefore, that we understand short-tailed hawk ecology and management needs on private lands if we are to maintain a viable state population. Even though most of the species’ winter distribution falls within the boundaries of public lands, their concentration during this season makes them vulnerable to natural and human perturbations of southern Florida’s narrow coastal zone. Short-tailed hawks wintering in the suburbs of Broward, Dade, and Monroe counties may benefit from the availability of exotic doves, upon which they seem to prey disproportionately. Hawks inhabiting suburbs, however, are more susceptible to human persecution (Millsap et al. 1996). Our present knowledge of the short-tailed hawk’s biology and conservation needs in Florida suggests that we adhere to the following course, which we pursued in our previous 2-year project: 1. Assess the present inventory of nesting areas and identify as many new sites as possible; 2. Focus on nesting birds to describe the associated foraging range, including habitat characterizations and range size; 3. Describe behavior and movements of dispersing young and the habitats occupied by wintering adults and young; and 4. Gather previously uncollected data on nesting ecology and demographics, including nesting success and productivity, the correlates of nesting success, survival, and age at first breeding. In early 1997, FWC’s Nongame Wildlife Program funded a 2-year project by Avian Research and Conservation Institute (ARCI), beginning in fall 1997, to develop study methods, including techniques for finding nests, and to begin gathering data on nesting and wintering ecology by focusing on radio-tagged adults and juveniles. That study was successfully completed and the present 2-year project began in late October 1999. The goals of the present study were to contribute to the database of known nest locations; to advance the methodology for population monitoring; to describe nesting, foraging,


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roosting, and overwintering habitats, thus improving Floridaâ&#x20AC;&#x2122;s ability to protect the species against the loss and degradation of critical habitat; and to collect reproductive and demographic data, which will be essential to modeling the population and predicting trends. The main change relative to the first study was to increase the field effort and to collect more data from radio-tagged birds. Short-tailed hawks are of the highest priority for research among Floridaâ&#x20AC;&#x2122;s vertebrates in part because so little work has been attempted, which is directly attributable to the challenges of studying this species: very small population size, inconspicuousness, use of remote areas with difficult terrain, wideranging movements, and difficulties related to capture. Although our sample sizes were relatively small, we are encouraged by the experience we gained. Virtually everything we learned is new and potentially valuable for managing this rare and vulnerable species.


SHORT-TAILED HAWK IN FLORIDAâ&#x20AC;&#x201D;Meyer

OBJECTIVES 1. Solicit sighting reports. 2. Trap and radio tag 6â&#x20AC;&#x201C;8 short-tailed hawks over 2 winters in southern Florida. 3. Use hawks radio tagged during winter and ground searches of historic and newly reported nesting areas to find nests during the breeding season. Find 8 nests over 2 nesting seasons (2000 and 2001). 4. Monitor nests to determine nesting success and productivity and to collect nest-site data. 5. Radio tag 8 nestlings over 2 years; trap and radio tag 3â&#x20AC;&#x201C;5 adults at their nests over 2 years. 6. Determine breeding-season foraging habitats and range sizes of radio-tagged adults; evaluate first-year survival and likely causes of death for radio-tagged young. 7. Determine winter foraging habitats and range sizes of radiotagged adults and young.

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METHODS Sighting Reports Reports were solicited from ornithologists, agency workers, and the birding public for any information related to the occurrence of short-tailed hawks in Florida. We used information from the sighting reports (not provided as part of this document) on a continuous basis over our 4 years of study to guide nest searching. Trapping and Radio Tagging During Winter In October 1999 we began monitoring sites in Everglades National Park and the Florida Keys where we or others had observed wintering short-tailed hawks. If hawks were regularly seen in specific accessible areas, we tried to trap and radio tag them. It is best to have a choice among several types of traps depending on the structure of the vegetation and the behavior of the hawks at a specific trapping site. One trap with which we had success was a portable bow-net (Tordoff 1954) with a remotely activated release and a tethered, movable Eurasian collared-dove (Streptopelia decaocto), house sparrow (Passer domesticus), European starling (Sturnus vulgaris), or rock dove (feral pigeon) (Columba livia) near the center as a lure. The lure was manipulated and the trap released from a nearby blind. Another trap consisted of the same moveable lure-bird arrangement but in conjunction with a pair of dho-gaza nets (2-m-square nets erected vertically and set to release when struck by a bird) placed at right angles to each other. Our least frequently used trap was the bal-chatri, a small wire mesh cage covered with monofilament nooses into which a starling or house sparrow was placed. Some of these traps were used in combination with each other or with a mist net placed nearby. All captured short-tailed hawks (i.e., adults and juveniles in the nesting and winter season) were handled, measured, blood-sampled, and fitted with transmitters in the same way. In November 2000 we deployed a satellite transmitter on an adult male short-tailed hawk (Sat 4) captured in Key Largo. All other captured hawks were fitted with VHF transmitters. Transmitters weighed <3% of birdsâ&#x20AC;&#x2122; body weight and were attached using a backpack harness. The harness, a proven design, was made of 6-mm-wide Teflon ribbon (Bali Ribbon, Bali, Pennsylvania). The ends of 4 separate strands were stitched together with cotton thread at a single point prior to capture and a 12mm square of soft material was threaded onto the Teflon to form a cushion at the junction. In the field, the junction was placed on the birdâ&#x20AC;&#x2122;s sternum and


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the free ends of the harness were brought up onto the bird’s back (2 strands anterior to the wings, 2 strands posterior to the wings and anterior to the legs), passed through holes at the front and back of the transmitter, tied, and stitched through the knot with nylon thread. We banded every bird we captured with a U.S. Fish and Wildlife Service aluminum band and took standard measurements (i.e., weight, wing chord, tarsus length and width, tail length, and culmen). We collected a small (0.10to 0.30-ml) blood sample from the brachial vein and stored it in lysis buffer. We isolated DNA from the blood and used it to determine the sex of each bird (polymerase chain reaction [PCR] analysis) (Norris-Caneda and Elliott 1998). The remaining DNA and blood samples were archived for future analyses. Each bird was released at the capture point, usually within 45–60 minutes of capture. Finding Nests We tried to track the radio-tagged wintering hawks as they departed the winter range during February and March of each year. First attempts were from the ground to minimize cost, but once the birds began moving, detection rates were poor. In the first 3 years of our study (the prior contract and half the present study), we flew 15–18 hours in spring and early summer to search for missing radio-tagged hawks. For the 2001 nesting season, FWC amended ARCI’s contract to increase the flying budget, which allowed us to fly 68 hours from February to July in search of nesting territories of radio-tagged short-tailed hawks. Relative to previous years, we flew more frequently, flew more closely spaced transects, and covered a wider area. We learned that a radio-tagged short-tailed hawk on, or perched near, a nest could be consistently detected from the air at a distance of 20–25 km. From there, we located the bird and mapped its position visually or with a Garmin 12x GPS receiver. Within 1–2 days, we tracked the signal from the ground. If the radio-tagged bird was a female, and it was early enough in the nesting cycle, the nest was readily found. If the bird was a male (i.e., less likely to be incubating or attending young) or if it was late in the season (when females are less attentive), it was necessary to make repeated visits and wait for the opportunity to track the adult into or out of the nest. By the time some radio-tagged birds were found from the air, insufficient time remained before the young fledged or the nest failed, in which case radiotracking provided no advantage in finding the nest. The radio equipment consisted of a programmable scanning VHF receiver with a 164.000–165.999 MHz range (model R-4000, Advanced Telemetry


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Systems, Isanti, Minnesota), wing-mounted 2-element yagi antennas (Telonics, Mesa, Arizona) connected to the receiver with coaxial cables, a switch box for selecting either or both antennas, headphones, and an intercom system to permit simultaneous monitoring of the receiver and communication with the pilot. We aerial tracked from a Cessna 172 or 152 single-engine highwing aircraft flown along transects 70–100 km apart at an altitude of 500– 700 m above ground level and an airspeed of about 180 km/hour. Previous experience demonstrated that these altitudes and flight speeds are optimal for signal detection and efficiency; combined with the VHF transmitters we used (American Wildlife Enterprises, Monticello, Florida), detection ranges were consistently >50 km and often >100 km. The location and orientation of the transects varied depending on the last known position of the subjects of our search. A GPS receiver was used to determine the coordinates of hawks as they were detected and to aid in navigation while flying transects. Monitoring Nests We tried to monitor nests weekly, but limited access to nesting areas and other logistical constraints sometimes precluded returning to nests at regular intervals. Typically, nest status was assessed from the ground by observing parental behavior. We considered birds to be on eggs if they were observed in a stereotypic incubation posture with head and torso low in the nest. We were able to view older nestlings (>2 weeks) from the ground with binoculars or a spotting scope to estimate their age. We estimated the beginning of the incubation and nestling periods through observations of parental behavior at the nest and by backdating from hatching or fledging date. For analyses, we used 32 days for the incubation period and 37 days for the nestling period. A successful nest fledged ≥1 young. We calculated nesting success rates with the Mayfield method (Mayfield 1961, 1975) as modified by Hensler and Nichols (1981). Nesting success rates for the incubation and nestling stages were calculated separately and then multiplied to determine overall success for the entire nesting period (Hensler and Nichols 1981, Hensler 1985). We compared nesting success during the incubation and nestling stages with 1tailed, standard normal Z tests (Hensler and Nichols 1981, Hensler 1985). Small samples precluded testing for annual differences in nesting success. Productivity was expressed as young fledged per attempt (nest with eggs) and young fledged per successful attempt. We visited nests regularly to monitor development of the young and to estimate when we could radio tag them. Nestlings had to be old enough to be


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properly fitted with a radio harness, but not so old that they fledged when the climber approached. Describing Nest-site Vegetation We collected the following vegetation data at 20 accessible nest sites for the nest tree and 5 randomly selected overstory trees within a 0.4-ha circular plot centered on the nest tree: species, height, dbh, crown height, crown width (longest and shortest axes), and basal area. For the nest tree we also measured height of the nest and adjacent crown closure. Trapping and Radio Tagging Adults and Young at Nests Young were captured in the nest at least 2 weeks prior to fledging (during May) by a climber who used a rope and mechanical ascenders to reach the nest. Nestlings were then lowered down to a team on the ground to take measurements and attach radio transmitters. We used an elevated mist net and a live owl as a lure to capture adults at nests. The mist net trap consisted of 2 2-x-12–m nets tied together along their length (making 1 4-x-12–m net) and attached at each end to a pulleyed rope arrangement suspended from the top of a 10-m extendable pole. The net was placed within 30–50 m of an active nest. A live disabled great horned owl (Bubo virginianus), which was borrowed from a wildlife rehabilitation facility, was tethered to a 2-m perch placed below the center of the net. When a hawk dove at the owl and was caught in the net, the net was quickly lowered and the hawk safely removed. Describing Summer Activity Ranges Breeding hawks carrying VHF transmitters were tracked from the ground during the incubation, nestling, and post-fledging stages, although at times a bird was out of range (usually because it had perched, in which case detection diminished). We timed our tracking exercises to span as long a period as possible, yet avoid temporal clustering. During each tracking period, the hawk’s location was estimated visually in relation to local features and plotted on 1:24,000 topographic maps. We also collected VHF summer locations opportunistically from the air during surveys for other projects. As with ground-based tracking, we timed the flights to cover as much of the nesting season as possible and to avoid clustering in time.


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We took VHF radio locations at no less than hourly intervals while hawks were on breeding territories. During this period we obtained radio locations opportunistically rather than on a stratified schedule. An observer would take a bearing on a bird from a known (GPS) location, and within 5 minutes take another bearing from a different location <1 km away. These bearings were mapped to triangulate on the hawk’s estimated location. All of the locations used in the activity range analyses were taken at intervals of at least 1 day and usually several days, assuring statistical independence for adjacent locations. During 2001, the satellite transmitter on Sat 4 provided an average of at least 1 useable location (i.e., of sufficient accuracy) every 30 hours. We used Service Argos (1996) Location Class (LC) data to decide whether each fix was sufficiently accurate for a particular type of analysis. Service Argos assigns 1 of 7 classes to each location, but we considered only the top 4 classes for inclusion in our analyses. They estimate that 67% (1 standard deviation) of the locations per class will be accurate to within the following distances: LC 3, ≤150 m; LC 2, 151–350 m; LC 1, 351–1,000 m; and LC 0, >1,000 m (Service Argos 1996). By scrutinizing each reported location in relation to prior and subsequent locations, the most blatant of these errors usually were obvious. After this process, we used all locations of LC 0 and better for mapping seasonal activity ranges. For vegetation analyses, we omitted LC 0 fixes. Useable satellite locations for Sat 4 were entered into an EXCEL spreadsheet. The locations were classified into 3 seasons (summer, early winter, and late winter) based on their geographic and temporal distribution. Datasets for each early and late winter season consisted of the combined locations for 2 winters, 1999-2000 and 2000-2001. We divided the winters into early and late stages because there was a very clear and consistent spatial and temporal distinction between the 2 periods in both winters. Working in ArcGIS, we categorized vegetation and land-use cover types at each telemetry point based on the Florida water management districts’ statewide 1995 land use/land cover raster grid of 298 classifications, reclassified by FWC into 44 types (R. Kautz, FWC, personal communication). To identify the cover type occupied by the hawk at each location, we brought the UTM coordinates for each location into the state land-cover map and converted each point to a grid cell equaling the pixel size for the habitat coverage (30 x 30 m). The habitat coverage cell values were labeled 1–44, representing each of the 44 land use/land cover types. Using the ArcGIS extension Spatial Analyst, hawk location grid cell values were then reclassified


SHORT-TAILED HAWK IN FLORIDAâ&#x20AC;&#x201D;Meyer

11

and placed over the habitat coverage as a mask, only allowing the grid cells of the hawk location to be represented. The Raster Calculator function was then used to add the masked habitat coverage back to the reclassified hawk grid to get a count of the hawk location cells that were represented in each cover-type classification. In order to determine the specific cover types (native vegetation and land use) significantly selected for or avoided by Sat 4, we chose random points from within a minimum convex polygon described by the hawkâ&#x20AC;&#x2122;s locations to represent available habitat. The number of random points for each of the 3 seasonal analyses equaled the number of actual hawk locations for that period. The rasterization, reclassification, and masking operations were repeated for the 3 sets of random points. The random locations were then evaluated in the same way as the actual hawk locations, determining the land cover type for each point. For each season, we compared the overall proportions of cover types of actual hawk locations (used) with random (available) points using G-tests (p < 0.05). For this test, we first collapsed similar cover types with fewer than 5 expected occurrences based on the proportions of those types in the available area and the total number of locations. We eliminated types that were neither used nor randomly selected in the available area. The net effect was that we included 28 cover types in the analysis. Significant results for a particular season indicated that, overall, the cover types were used in disproportion to their availability. To identify the specific cover types used significantly more or less than expected relative to the available area, we set 95% confidence limits around the actual (used) proportions for each cover type. To calculate the Z score, we adjusted the alpha level for multiple simultaneous comparisons by dividing alpha by 2k, where k = number of cover types. If the random (available) proportion for a specific cover type fell below the lower limit of the confidence interval, we concluded that this cover type was significantly selected by the hawk. If the random proportion fell above the upper limit, there was significant avoidance of that cover type. We mapped and estimated the maximum sizes of activity ranges for VHF radio-tagged hawks using the 100% minimum convex polygon (MCP) method (Mohr 1947) of the Animal Movements extension (Hooge and Eichenlaub 1997) of ArcView (Earth Systems Research Institute 1996). The small number of VHF radio-locations collected for most individuals precluded the use of nonparametric techniques that might better identify the size and shape of the central activity area utilized in normal movements (Jennrich and Turner 1969, White and Garrott 1990).


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In analyzing the larger data sets for seasonal activity ranges of the satellite-transmittered hawk, we limited our descriptions of activity ranges to the MCP. The data probably are sufficient to apply the Kernel method as well, which would highlight concentrated areas of use. It is relatively easy when viewing the activity range maps based on satellite telemetry, however, to determine, based on the distribution of hawk locations within the MCP, the most likely core areas and their relationships with vegetation. Describing Winter Activity Ranges Radio-tracking and activity-range analysis methods for wintering hawks with both VHF and satellite transmitters were the same as those used for the breeding season (described in the previous section), except that the tracking period was longer (2- to 7-day intervals) and unspecified for wintering hawks. We also relied more on aerial tracking than ground tracking to collect winter data. Tracking times, however, were still stratified to the extent possible within days and weeks over the course of the season. Estimating Survival Our proposed method for estimating annual survival rates of radio-tagged short-tailed hawks was the product-limit estimator procedure (Kaplan and Meier 1958), modified to allow for the staggered entry of new radio-marked animals added to the sample population as the study progressed (Pollock et al. 1989). The product-limit estimator allows for the removal or censoring of animals over time when their radio transmitters fail. High survivorship and our small sample, however, made this analysis unnecessary.


SHORT-TAILED HAWK IN FLORIDAâ&#x20AC;&#x201D;Meyer

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RESULTS Trapping and Radio Tagging During Winter Trapping success improved in the winter of 1999-2000 over the previous project, with more birds caught in this single season than in the 2 previous winters combined. From 27 October to 20 December 1999, we captured and radio tagged 9 wintering short-tailed hawks and radio tagged a bird about to be released by a wildlife rehabilitation facility after recovering from injuries (a total of 4 after hatch year [AHY] and 6 hatch year [HY] birds) (Table 1). During the winter of 2000-2001 we captured and radio tagged 9 short-tailed hawks from 25 October to 17 December; a tenth hawk was captured and radio tagged for us by the Florida Keys Raptor Migration Project in Tavernier (a total of 6 AHY and 4 HY birds) (Table 1). Thus, with 20 birds marked, we attained more than twice our goal of tagging 6â&#x20AC;&#x201C;8 birds during 2 winters. Of the 19 free-ranging birds that were captured, 8 were lured with European starlings, 6 with Eurasian collared-doves, 4 with house sparrows, and 1 with a rock dove. Fifteen were captured in the Florida Keys and 4 in Everglades National Park. We made standard measurements on all short-tailed hawks radio tagged during this period (Table 2). Finding Nests We found and monitored 27 nests over 4 years (Table 3). Seventeen of the nests were found during the last 2 years, exceeding the proposed objective of locating 8 nests during this period. The 27 nesting attempts represented 14 individual territories in 10 counties in peninsular Florida (Fig. 1). Twelve nests were in loblolly pines (Pinus taeda), 7 in red bays (Persea borbonia), 6 in bald cypress (Taxodium distichum), 1 in a red maple (Acer rubrum), and 1 in a black mangrove (Avicennia germinans). Seven of the 14 territories were on privately owned lands, 5 were on public lands, and 2 were on private conservation lands (The Nature Conservancy). Of the 7 privately owned parcels, 5 were managed primarily for timber and 2 for some combination of grazing, hunting, and timber. Our nest-finding success was not evenly distributed over the 2 years of the study. In 2000, as in the previous 2-year project, we hoped to gain some advantage by searching for hawks radio tagged prior to the 2000 nesting season, but this was not the case. The transmitters on the Lake Pierce breeding male and female (nests found in 1998 and 1999) expired prior to the 2000


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Table 1. Short-tailed hawks (n = 40) radio tagged in southern Florida during the present and previous studies (1997â&#x20AC;&#x201C;2002). All except one were fitted with VHF transmitters; Sat 4 was tagged with a satellite transmitter. Bird #

Trap location

County

Date tagged

Agea

Sex

Morph

5.777 5.886 5.382 5.576 4.435 4.464 4.448 4.515 4.557 4.500 4.574 4.481 4.737 4.677 4.585 4.649 4.767 4.692 5.074 4.629 4.709 5.111 5.731 5.787 5.807 5.829 5.788 Sat 4 5.926 5.935 5.913 5.845 5.854 5.865 5.582 4.098 4.119 4.141 5.558 5.058

Flamingo Marina Rehab #1 Lk. Pierce Lk. Pierce Lk. Pierce Arbuckle Rehab #2 Hidden Lake Rehab #4 Arbuckle Arbuckle Arbuckle Flamingo Marina Curry Hammock Trailer Loop West Lake Key Largo 1 Key Largo 2 Key Largo 3 Key West 98 Key West 99 Hollywood Waccasassa Rehab #5 Arbuckle Long Key Rudy 1 Rudy 2 Flamingo Camp Cudjoe 1 Cudjoe 2 Rudy 3 Pirate/Cudjoe 3 Rudy 4 Rudy 5 Deer Park Tiger Creek Key West 3 Tiger Creek Red Beach Lake

Monroe Polk Polk Polk Polk Highlands Dade Monroe Broward Highlands Highlands Highlands Monroe Monroe Monroe Monroe Monroe Monroe Monroe Monroe Monroe Dade Levy Polk Highlands Monroe Monroe Monroe Monroe Monroe Monroe Monroe Monroe Monroe Monroe Pasco Polk Monroe Polk Highlands

11/20/1997 3/20/1998 6/2/1998 6/2/1998 6/24/1998 7/2/1998 9/7/1998 10/30/1998 2/11/1999 5/13/1999 5/13/1999 5/20/1999 10/27/1999 10/28/1999 10/29/1999 11/5/1999 11/17/1999 11/18/1999 11/18/1999 11/20/1999 11/20/1999 12/20/1999 4/25/2000 5/27/2000 6/1/2000 10/17/2000 11/20/2000 11/21/2000 11/28/2000 11/30/2000 11/30/2000 12/4/2000 12/6/2000 12/12/2000 12/13/2000 5/23/2001 5/27/2001 12/14/2001 6/3/2002 6/3/2002

HY AHY AHY AHY HY HY AHY HY AHY HY HY AHY AHY HY HY HY HY HY AHY AHY HY AHY AHY AHY AHY AHY HY AHY AHY HY HY HY AHY AHY AHY HY HY HY HY HY

unk F M F M M F F F M M M F unk M F M F F F M M M F M F M M F F F F F F F M M F F M

Light Dark Dark Dark Light Dark Light Dark Light Dark Dark Dark Dark Light Dark Light Light Dark Light Dark Dark Dark Light Light Dark Dark Dark Dark Light Light Light Light Dark Dark Dark Dark Dark Dark Dark Light

aAHY

= after hatch year, HY = hatch year.


SHORT-TAILED HAWK IN FLORIDAâ&#x20AC;&#x201D;Meyer

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Table 2. Measurements of 40 short-tailed hawks captured in southern Florida from November 1997 to June 2002.

Bird #

Weight (g)

Tarsus (mm)

Wing (mm)

Tail (mm)

Culmen (mm)

4.464 4.500 4.574 4.481 5.807 5.935 5.913 4.677 4.098 5.926 4.737 5.777 4.515 5.111 4.767 5.074 4.692 4.629 4.141 4.709 5.576 5.382 4.435 5.829 5.854 5.886 4.448 4.557 5.787 5.788 Sat 4 5.845 5.865 5.582 4.119 4.585 5.731 4.649 5.558 5.058

430 410 420 385 465 625 535 588 435 580 628 575 710 408 390 600 585 450 650 435 565 385 400 635 615 659 495 495 495 400 444 505 590 625 390 480 450 560 569 385

7.2 7.5 6.4 7.1 7.0 6.8 6.8 7.4 7.1 7.6 6.7 7.1 7.4 6.5 6.0 8.7 8.1 6.4 8.1 7.1 8.3 6.0 7.1 8.4 7.2 Unk 6.4 6.4 6.0 5.4 5.5 6.7 6.4 6.4 6.0 6.7 5.5 7.5 6.5 6.2

187 217 310 308 296 340 329 319 Unka 337 332 320 333 305 309 336 331 330 324 306 340 305 315 343 329 342 329 334 304 319 304 326 332 331 Unk 309 305 322 Unk Unk

70 103 171 157 156 190 178 181 Unk 180 171 155 174 157 165 177 187 176 184 163 178 156 167 186 174 176 171 169 161 165 164 175 171 159 Unk 164 141 176 Unk Unk

16.0 16.1 16.9 17.5 12.0 20.0 19.0 21.6 15.0 19.7 19.0 20.8 19.9 18.5 17.3 19.3 19.7 broken 19.0 17.5 18.0 17.4 16.7 20.7 19.0 19.0 20.6 21.1 19.0 18.0 18.0 19.0 20.0 20.0 16.0 17.8 17.5 20.0 20.0 15.0

aWing

and tail measurements were not taken on nesting birds.


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

Table 3. General locations of 27 short-tailed hawk nests found in southern Florida and monitored during 4 breeding seasons, from 1998 to 2001 (present and previous studies combined). These 27 nesting attempts represent 14 individual territories in 10 Florida counties. Year

Location/name

County

1998

Arbuckle Creek Lake Pierce Walk-in-Water Arbuckle Creek Lake Pierce Walk-in-Water Lake Patrick Wacassassa Kissimmee Blue Cypress Arbuckle Creek Walk-in-Water Lake Patrick Waccasassa Tiger Creek Arbuckle Walk-in-Water Lake Patrick Waccasassa Tiger Creek Blue Cypress Deer Park Cow Heaven Shell Mound Weeki Wachee Pirate Cape Sable

Highlands Polk Polk Highlands Polk Polk Polk Levy Okeechobee Indian River Polk Polk Polk Levy Polk Polk Polk Polk Levy Polk Indian River Pasco Putnam Levy Hernando Osceola Monroe

1999

2000

2001

aApproximate

Coordinates (decimal degrees) 27.8 x 81.5 28.0 x 81.7 27.8 x 81.5 27.4 x 81.3 28.0 x 81.7a 27.8 x 81.5 27.8 x 81.5 29.2 x 82.8 27.4 x 81.1a 27.7 x 80.8a 27.4 x 81.3 27.8 x 81.4 27.8 x 81.5 29.2 x 82.8 27.8 x 81.5 27.4 x 81.3 27.8 x 81.4 27.8 x 81.5 29.2 x 82.8 27.8 x 81.6 27.8 x 81.5 28.2 x 82.7 29.6 x 81.8 29.2 x 83.0 28.6 x 82.7 28.3 x 81.5 25.2 x 81.1

location derived from a map because we had no access to nest site.

nesting season. The same was true for 5 other radio-tagged hawks that would have been â&#x2030;Ľ2 years old in the spring of 2000. Of the 13 remaining radiotagged hawks, 8 were <2 years old in March 2000 and were not expected to be breeding. Although we used all affordable flight time searching for the 5 birds of potential breeding age, we only found 1, and that bird did not nest. The other 4 were â&#x2030;Ľ2 years old and could have nested. The only radio-tagged adult with an operating radio among those breeding on the above territories was male #5.480. He was already present on his 1999 territory when we first checked the site on 6 March 2000. He remained there until late March, but he was never observed with another short-tailed hawk. We were unable to locate a nest structure until 12 April 2000, after a dark morph male without a transmitter was repeatedly seen in the area with a presumed mate. It was not until this male was captured on 1 June 2000,


SHORT-TAILED HAWK IN FLORIDAâ&#x20AC;&#x201D;Meyer

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Fig. 1. Locations (dots) of 27 short-tailed hawk nesting attempts in Florida from 1998 to 2001. Some closely adjacent nests built on the same territory in different years appear as a single point at this viewing scale.


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

however, that we were able to confirm that it was not the 1999 breeding male (#4.481, also dark). Hawk #4.481 was never found during any of our statewide searches, but there is no question that he was replaced on the Arbuckle Creek territory in 2000. Had we not radio tagged #4.481, we would have assumed that the 1999 and 2000 breeding adults were the same bird. As time permitted, we searched 9 additional areas (in Dixie, Okeechobee, Polk, Highlands, and Orange counties) where we or others had observed hawks during a previous nesting season or where a radio-tagged hawk of unknown breeding status had been detected more than once during the current nesting season. No nests were found in any of these places. Prior to the 2001 nesting season, FWC agreed to amend the contract with additional funds for flight time, with the goal of improving our nest-finding success. This allowed us to expand our efforts well beyond the known nesting areas studied in previous years and addressed the concern that our nest searching may have been biased toward relatively open areas in which the hawks were more conspicuous and where nest stands were smaller, more accessible, and more readily searched. In the 2001 breeding season, we continued to search previously used areas (Arbuckle Creek, Walk-in-Water, Lake Patrick, Waccasassa, and Lake Pierce territories) without the benefit of having radio-tagged nesting females to track to nests (adult male #5.807 at Arbuckle Creek was radio tagged, but tracking him provided no advantage for finding the nest). As expected, nest finding progressed very slowly, even though we were working in relatively small stands with good observation points. Arbuckle Creek and Walk-in-Water were active in 2001, and we found both nests by April. We were once again denied property access at Lake Pierce, but inferred nesting activity from behaviors observed from an adjacent pasture. Lake Patrick and Waccasassa were occupied by pairs in March but both sites were abandoned by April. Both of these territories produced eggs in 1999 and 2000 but all 4 attempts failed no later than the early nestling stage and probably during incubation. In addition to Lake Arbuckle and Walk-in-Water, 2 other territories were confirmed active in 2001: Tiger Creek, which was active in 2000, and Deer Park, a new site in Pasco County. At all 4 confirmed sites, and at the unconfirmed Lake Pierce site and the 2 abandoned sites (Waccasassa and Lake Patrick), hawk nesting behaviors could be observed from easily accessed points within a few hundred meters of the nestsâ&#x20AC;&#x201D;sufficient evidence to trigger intensive ground-searching for the nest. With the advantage of an increased flying budget, however, we also focused intensively on tracking females at least 2 years old that had been radio tagged during the 2 previous winters in


SHORT-TAILED HAWK IN FLORIDA—Meyer

19

southern Florida. We hoped to increase our catalog of confirmed nesting territories and avoid the problem of a bias toward finding nests in relatively small, accessible stands when only searching visually and from the ground. We increased our aerial search time to 68 hours from March to June. As a result, we detected 20 of 25 hawks known to be alive and carrying functioning transmitters as of January 2001. Of the 20, 2 (adult males #5.807 and #5.731) were associated with previously discovered territories. Eleven were yearlings, which we assumed were not breeding, or adult males, which are much more mobile than females during the incubation stages. We gave priority to tracking the 7 radio-tagged females that were ≥2 years old. We intensified our aerial searches for these females and found that, in all cases, they were consistently located in the same small area during successive flights, appeared not to be flying, and could be detected from no more than 20–25 km, whereas signals of hawks in flight can be heard consistently at 30–90 km or more. In 6 cases, we eventually confirmed nesting by the females. In the seventh case, at Hontoon Island, the bird (#4.677) was almost certainly nesting but we could not confirm this visually and locate the structure before she began spending her time away from the nest, either because her young had grown or the nest had failed. The 6 confirmed new nest sites (with the female’s ID number in parentheses) were Shell Mound (#5.865), Weeki Wachee (#5.926), Shingle Creek (#5.854), Cow Heaven (#5.582), Blue Cypress Lake (#4.649), and Cape Sable (#5.074). The landscapes surrounding the 7 territories found solely as a result of radio tracking differed from those associated with territories found visually from the ground. As with the visually located territories, all were in mature, mixed-species, swamp forest. The sizes of the 7 nest stands, however, and the locations of the nest within the stand for 6 of the territories were quite different. Six were well within the interior of very large, continuous expanses of closed-canopy forest (e.g., Chassahowitzka Swamp for the Weeki Wachee site, Black Point Swamp for Shell Mound). The seventh site, Shingle Creek, would have fit this description had the area immediately adjacent to the nest not been recently clear-cut (we could not determine exactly when). All 7 sites were remote from human activity and disturbance. Monitoring Nests Of the 22 nests at which fate could be determined unambiguously, 9 were successful (i.e., ≥1 young) (Table 4).


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The traditional (uncorrected for observation period) estimate of nesting success was 41%. The Mayfield estimate was 37%. These results and the supporting statistics (Table 5) indicate that nests were much more likely to fail during the egg stage than the nestling stage. Six of the 13 failures were at 2 territories, each monitored 3 successive years. In all 6 cases, an adult was observed incubating for at least 1 week beyond the expected hatch date (usually more than 2 weeks beyond). At 5 of the 6 other failures, hatching never occurred despite a more than adequate incubation period. Productivity for the 22 nests was 0.64 young per attempt (nest with eggs) and 1.56 young per successful attempt (Table 4). Because our radio transmitters operated for no more than 27 months, it was difficult to confirm exact age of first breeding for birds tagged as nestlings or during their first winter, when their plumage provided definitive evidence of their age. Our limited opportunities, however, were very informative. Two HY females (#4.677 and #4.649) trapped and radio tagged their first winter,

Table 4. Success and productivity of 22 short-tailed hawk nests in southern Florida for which fate could be determined, monitored from 1998 to 2001 (present and previous studies combined). Successful nests fledge â&#x2030;Ľ1 young. Year

Location/name

Successful

Number of young fledged

1998

Arbuckle Creek Lake Pierce Walk-in-Water Arbuckle Creek Walk-in-Water Lake Patrick Wacassassa Arbuckle Creek

Yes Yes No Yes No No No No

2 2

1999

2

Table 5. Summary statistics for Mayfield and traditional analyses of nesting success for 22 breeding attempts of short-tailed hawks in Florida from 1998 to 2001 (present and previous studies combined).

Total exposure days Estimated daily survival: p Standard deviation of p Nesting success (Mayfield) Standard deviation of nesting success Traditional nesting success

Incubation

Nestling

422 0.98 0.01 0.46 0.11 0.47

331 0.99 0.01 0.80 0.02 0.82

Overall 743 0.37 0.10 0.41


SHORT-TAILED HAWK IN FLORIDA—Meyer

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attempted to nest for the first time when 2 years old. Both attempts failed. A female (#5.829) radio tagged in the fall of 2000 as an AHY bird did not attempt to breed the following spring (2001), when she had to have been ≥2 years old. She nested successfully in 2002, when ≥3 years old. Similarly, a male (Sat 4) trapped and radio tagged in the fall of 2000 as an AHY bird did not attempt to breed the following spring (2001), when he had to have been ≥2 years old. He attempted to nest in 2002, although we were unable to determine the outcome. The female and male marked as adults may have been >2 years when captured and may have foregone nesting the subsequent spring for reasons other than sexual immaturity. The case for the 2 females caught as HY birds nesting at 2 years old, however, is unambiguous. Nest Site Vegetation Table 6 includes the measurements of nest trees, random overstory trees, basal area, and crown closure of 0.40-ha circular plots centered on the nest trees.

Table 6. Vegetation measurements (means ±1 standard deviation) within 0.4-ha circular plots surrounding 20 short-tailed hawk nest trees in 4 breeding season from 1998 to 2002 (present and previous studies combined).

Variable Nest height (m) Tree height (m) Dbh (cm) Crown height (m) Crown area (m2) Basal area (m2/ha) based on factor 10a Crown closurea

Nest tree (n = 20) 20.1 ± 2.6 23.5 ± 2.8 47.6 ± 7.4 9.8 ± 2.5 40.8 ± 23.0 23.7 ± 12.8 74.3% ± 19.7%

Random overstory tree (n = 100) n/a 20.3 ± 3.8 34.9 ± 15.4 7.2 ± 2.8 39.6 ± 41.3

aBasal area and crown closure were estimated from measurements taken throughout the stand, at the nest tree and random overstory trees.

Trapping and Radio Tagging Adults and Young at Nests The best way to radio tag young short-tailed hawks is to take them from their nests when they are large enough to be safely fitted with the transmitter harness, yet not old enough to fledge as the climber approaches. Recently fledged young return to the nest regularly to feed and roost, and we captured


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

2 such birds with a noose carpet placed over the nest. This method is potentially dangerous, however, for both bird and climber, and young hawks often would not land on a nest with a noose carpet. We were unable to trap recently fledged short-tailed hawks by any method once they stopped returning to the nest to feed or roost. The best method for radio tagging young of the year was to capture them in the nest prior to fledging. Difficulties included timing the climb within the narrow time period when the nestling could be harnessed, low nesting success (i.e., few young to choose from), condition of the nest tree, and location of the nest within the tree. We captured and radio tagged 2 nestlings in the course of the present study plus 2 more in the 2002 nesting season (data included in this report). The proposed objective was to radio tag 8 nestlings during the present study. We radio tagged 8 over all 4 years of research. We captured 2 breeding adults at nests and fitted them with VHF transmitters during the present study. A light morph adult male (#5.731) on the Waccasassa territory was captured on 25 April 2000. The trap, a dho-gaza net with a house sparrow as a lure, was set in a cut-over area within 60 m of the edge of the nest-tree stand, about 200 m from the nest. We used playbacks of bird distress calls (potential prey) to attract the hawk. The Waccasassa male was captured the second day we set this trap (first set on 20 April 2000). This is the first nest at which we tried to trap a nesting adult away from its nest using a prey bird as a lure. The second adult (breeding male #5.807) was captured on 1 June 2000. The trap was a 12-x-8–m mist net with a live great horned owl as a lure. This was the same arrangement we usually employed near nests. In this case, however, the trap was set 150–200 m from the nest, across Arbuckle Creek on an open peninsula, where breeding male #4.481 was captured in 1999. A third adult (#5.787), a captive bird that had been treated for a broken wing at the Miami Museum of Science and held for 2 years, was radio tagged and released on 27 May 2000 near Lake Walk-in-Water. The remains of this bird were recovered within 12 hours of death on 16 June 2000 within 10 km of where it was released. It appeared to have starved to death. The proposed objective was to radio tag 3–5 breeding adults during the study. We radio tagged 5 over the 4 years of our research.


SHORT-TAILED HAWK IN FLORIDA—Meyer

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Summer Activity Ranges Of the 126 locations produced by VHF-tagged hawks in the summer, 105 (83%) were from the ground and 21 (17%) were from the air. We directly observed the bird at 55 (44%) of the 126 ground locations. We triangulated on the remaining 71 ground locations from either 2 or 3 positions. The triangulations were simultaneous in 29 (41%) of the cases, all taken from 2 positions. The other 42 triangulations were made by 2 observers from 2 or 3 positions within a 5-minute period. We determined the 100% MCP areas and perimeters for 11 radio-tagged short-tailed hawks in summer (8 birds) and winter (9, including some in the summer sample) (Table 7). The smallest activity ranges were those of breeding adults (both sexes) during summer. The largest activity ranges were those of hawks that had not yet bred, both during summer and winter. The summer ranges of 3 nesting adults (males #5.731 and #5.807 and female #5.576) (Fig. 2) contrast with those of #4.464, #4.585, and #4.500, all nonbreeding male HY birds (Figs. 3, 4, and 5). As some of the maps indicate, however, the large areas of the 100% MCPs result from relatively few outlying points. Much smaller core areas are evident

Table 7. Areas and perimeters of activity ranges of 11 short-tailed hawks estimated using 100% minimum convex polygons. The first 10 hawks were tagged with VHF transmitters; the last hawk carried a satellite transmitter.

Bird #

Season

Dates tracked

4.464

Summer Winter Summer Winter Winter Winter Summer Winter Winter Summer Winter Summer Summer Summer Early winter Late winter Summer

3/26/99–10/26/99 10/22/98–3/4/99, 12/1/99–2/4/00 2/3/00–11/16/00 10/26/99–3/31/00 11/1/98–3/4/99, 5/14/99–1/12/00 9/22/99–2/26/00, 11/16/00–1/11/01 4/18/00–5/13/00, 3/9/01–8/8/01 11/19/99–3/31/00, 10/19/00–2/15/01 11/17/99–4/18/00, 10/19/00–4/19/01 6/3/98–10/25/98, 3/16/99–10/26/99 11/11/98–12/13/98 6/3/98–10/25/98 4/30/00–9/30/00, 2/23/01–8/17/01 6/2/00–6/12/00, 1/11/01–8/8/01 11/22/00–12/11/00,10/28/01–12/9/01 12/12/00–3/13/01, 12/10/01–12/30/01 4/6/01–10/25/01

4.500 4.515 4.574 4.585 4.767 5.382 5.576 5.731 5.807 Sat 4

Number of locations

Area (km2)

Perimeter (km)

20 18 9 7 47 11 13 17 15 23 10 26 19 20 47 90 97

3,843 2,492 400 6,297 1,260 3,407 2,367 3,058 3,089 12 245 12 108 844 4,760 4,576 10,467

269 227 81 415 166 309 200 213 244 13 69 5 73 151 292 304 404


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

(e.g., Figs. 3 and 4). This was less the case for summering birds, which tended to use their ranges more uniformly, even though they were about as large as their winter ranges (Figs. 3, 4, and 5). To some extent, the inflated range polygons for wintering hawks are the result of the less continuous distribution of forested habitats, the preferred cover types, in southern Florida (see habitat analyses for Sat 4, below). Large intervening expanses of freshwater marsh, which the hawks avoided, occupy much of the area enclosed by the MCPs describing ranges of wintering hawks. This was not the case for ranges of summering hawks.

Fig. 2. Summer activity ranges, derived from ground and aerial VHF radio-tracking, of 3 breeding adult short-tailed hawks. Black lines delineate100% minimum convex polygons. The range in Levy and Citrus counties (on Waccasassa Bay) is that of male #5.731, tracked 4/30–9/30/00 and 2/23–8/17/01. The range in Polk County (on the Lake Wales Ridge northwest of Lake Kissimmee) is that of female #5.576, tracked 6/3–10/25/98. The range in Highlands, Okeechobee, and Glades counties (including Lake Istokpoga) is that of male #5.807, tracked 6/2–6/12/00 and 1/11–8/8/01.


SHORT-TAILED HAWK IN FLORIDA—Meyer

25

Fig. 3. Summer (3/26–9/26/99) and winter (10/22/98–3/4/99 and 12/1/99–2/4/00) activity ranges, derived from ground and aerial VHF radio-tracking, of juvenile male short-tailed hawk #4.464. Black lines delineate100% minimum convex polygons.


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

Fig. 4. Summer (4/18–5/13/00 and 3/9–8/8/01) and winter (11/19/99–3/31/00 and 10/19/00–2/15/01) activity ranges, derived from ground and aerial VHF radio tracking, of juvenile male short-tailed hawk #4.585. Black lines delineate100% minimum convex polygons.


SHORT-TAILED HAWK IN FLORIDA—Meyer

27

Fig. 5. Summer (2/3–11/16/00) and winter (10/26/99–3/31/00) activity ranges, derived from ground and aerial VHF radio-tracking, of male juvenile short-tailed hawk #4.500. Black lines delineate100% minimum convex polygons.


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Hawk #5.382, a breeding male that attempted to breed in all years of the study, had the smallest winter and summer ranges of any of the hawks in the analysis (Fig. 6). The spring migration (3/16/01–4/6/01) and summer range (4/6/01– 10/25/01) of Sat 4 showed a lack of focus during the first year it was tracked, suggesting that the bird did not nest in 2001. Sat 4’s activity was much more concentrated in the second year, from fall 2001 through 15 September 2002, which included his first nesting attempt (Fig. 7). The 100% MCP area in 2001 (Fig. 8) was 10,467 km2 (Table 7). The use-versus-availability analysis of vegetation and land use cover types based on satellite data for Sat 4 during summer 2001 did not identify any cover types used in disproportion to availability (Table 8). Winter Activity Ranges Although of comparable size, summer ranges of nonbreeding males #4.464, #4.585, and #4.500 (Figs. 3, 4, and 5) were used more uniformly than winter ranges. Winter ranges of these 3 AHY hawks show the same patterns as for those of HY hawks #4.767, #4.574, and #4.515 (Fig. 9): core areas with scattered outlying points and large intervening expanses of unforested, unused habitat.

Table 8. Vegetation and land use cover types significantly selected for and avoided by short-tailed hawk Sat 4, derived from satellite telemetry locations over a 16-month period from November 2000 to March 2002. For the 2 seasons (early and late winter) for which G tests revealed selection, significant differences between proportions of used cover types versus available cover types were determined by comparing the available proportion for each cover type with the 95% confidence interval of the used proportion. Season

Cover type

Early winter

Mixed hardwood swamp Mangrove swamp Water Freshwater marsh Cypress-gum swamp Mixed hardwood swamp Agriculture, crops Freshwater marsh

Late winter

a–

= avoided, + = selected. = approached significance, < 0.01.

b(+)

Usagea + + – – + (+)b – –


SHORT-TAILED HAWK IN FLORIDA—Meyer

29

Fig. 6. Summer (6/3–10/25/98 and 3/16–10/26/99) and winter (11/11–12/13/98) activity ranges, derived from ground and aerial VHF radio-tracking, of adult male short-tailed hawk #5.382. Black lines delineate 100% minimum convex polygons. Due to overlapping points the breeding range appears as a small, solid area in Polk County (on the Lake Wales Ridge northwest of Lake Okeechobee).


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

Fig. 7. Seasonal locations derived from satellite telemetry for adult male short-tailed hawk Sat 4 during 2 years of tracking from November 2000 through 15 September 2002. In 2000 he moved from his early winter range to late winter range on 11 December. He did not nest during summer 2001. In 2001 he moved from his early winter range to late winter range on 10 December. He nested in the Wekiwa River basin (concentration of dots in Seminole County) during summer 2002.


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Fig. 8. Summer (4/6–10/25/01), early winter (11/22–12/11/00 and 10/25–12/9/01), and late winter (12/12/00–3/13/01 and 12/10/01–2/26/02) activity ranges, derived from satellite telemetry, of adult male short-tailed hawk Sat 4 in 2000 and 2001. Black lines delineate100% minimum convex polygons.


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During 2 years of tracking, Sat 4 used 2 spatially disjunct winter ranges (Fig. 7). The respective early and late winter ranges were remarkably similar between years, as was the timing of the hawk’s move between the 2 areas. During the early winters of 2000 and 2001, the 100% MCP range area (Fig. 8) was 4,760 km2 and the perimeter was 292 km (Table 7). The late winter activity range area (Fig. 8) was 4,576 km2 and the perimeter was 304 km (Table 7). The pattern among VHF radio-tracked birds also is seen in the 100% MCP for Sat 4 during all 3 seasons—summer, early winter, and late winter (Fig. 8). Use of the summer range was more uniform than that of the winter ranges, where core areas, outlying locations, and large intervening expanses of unused habitat were apparent. Sat 4’s preference for forested habitats is particularly evident during winter.

Fig. 9. Winter activity ranges, derived from ground and aerial VHF radio tracking, of 3 hatch-year shorttailed hawks. Black lines delineate100% minimum convex polygons. Hawk #4.515 was tracked in winter 1998/99 and 1999/2000; #4.767 and 4.574 were tracked in winter 1999/2000 and 2000/01.


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The use-versus-availability analysis of vegetation and land use for the combined early and late winter periods of 2000 and 2001 revealed that Sat 4 used wetland forests (mixed species hardwood swamp, cypress-gum swamp, and mangrove swamp) more than expected relative to availability, while open water, freshwater marsh, and agricultural fields were used less than expected (Table 8). Survival The small sample size and short monitoring period for most of the marked birds precluded statistical analysis. Of 8 birds VHF radio-tagged as nestlings, 1 died within 3 weeks of fledging, 2 survived at least through the first 6 months after fledging, 2 survived ≥18 months after fledging, and 3 survived ≥2 years after fledging, at which point their transmitters expired. Sex and Color-morph Ratios About two-thirds of the trapped birds were dark and one-third were light (Table 9), which is generally what has been reported for the Florida population (Miller and Meyer 2002). Our samples sizes limited comparisons by age and sex classes. There is some suggestion, however, that the ratio of dark:light birds may increase with age for both sexes. Three of the 4 possible pairing combinations of color morphs were represented among the breeding adults (Table 10). Of 11 breeding males, 73% (8) were dark. Of 8 adult males trapped, 75% (6) were dark. Of 15 breeding females, 73% (11) were dark. Of 13 adult females trapped, 62% (8) were dark. Thus, neither color morph seems to offer an advantage for either sex with regard to obtaining a mate. Ten dark females paired with 7 dark (70%) and 3 Table 9. Color morphs of 39 short-tailed hawks captured in southern Florida from 20 November 1997 through 3 June 2002. Dark Adult male (n = 6) Adult female (n = 13) Juvenile male (n = 9) Juvenile female (n = 8) Juvenile, sex unknown (n = 3) All adults (n = 19) All juveniles (n = 20) All males (n = 15) All females (n = 21) All hawks (n = 39)

5 (83%) 8 (62%) 6 (67%) 4 (50%) 1 (33%) 13 (68%) 11 (55%) 11 (73%) 12 (57%) 24 (62%)

Light 1 (17%) 5 (38%) 3 (33%) 4 (50%) 2 (67%) 6 (32%) 9 (45%) 4 (27%) 9 (43%) 15 (38%)


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

light males, again reflecting availability (75% of adult males were dark based on trapping). Of 8 dark males, 7 (86%) paired with dark females and 1 with a light female (62% of trapped females were dark). It appears that short-tailed hawks do not select mates with regard to color morph, either as an individual preference or in relation to their own type. The sex ratio of trapped adults in our small sample was skewed toward females, 8:5 (Table 11). The same was true to a smaller degree for juveniles (birds during their first year of life). This may reflect a trapping bias instead of, or in addition to, a natural skew in sex ratio, although it is not immediately clear why we should have been more likely to capture females.

Table 10. Color morph of each sex in 15 pairs of nesting short-tailed hawks observed in southern Florida from 1998 to 2001. For territories used repeatedly, only 1 pair is included because the color combination of male/female did not change (individual birds may have changed between years, but we had no way of knowing). The exception is Arbuckle 1998 and 1999, assumed to be 1 pair, and Arbuckle 2000-2001, known to have at least a different male than in 1998 and 1999 because both males were radio tagged.

Territory name

Years active

Color of male

Color of female

Walk-in-Water Lake Pierce Arbuckle Lake Patrick Waccasassa Arbuckle Deer Park Shingle Creek Cow Heaven Blue Cypress Weeki Wachee Cape Sable Tiger Creek Shell Mound Red Beach

1998–2002 1998 1998–1999 1999–2001 1999–2001 2000–2001 2001 2001 2001 2001 2001 2001 2002 2002 2002

Dark Dark Dark Dark Light Dark Light Dark Unknown Unknown Unknown Unknown Dark Dark Light

Light Dark Dark Dark Dark Dark Dark Dark Dark Light Light Light Dark Dark Dark

Table 11. Known sexes of short-tailed hawks captured in southern Florida from 1998 to 2002 (all rehabilitated birds [n = 6] were removed). All adults and juveniles (fully fledged young of the year) in this sample were caught either during winter or at their nests. All nestlings were captured in nests, usually at 22–26 days of age (≥1 week before fledging). Age Adult Juvenile Nestling Total

Male

Female

Total

5 4 7 16

8 7 1 16

13 11 8 32


SHORT-TAILED HAWK IN FLORIDAâ&#x20AC;&#x201D;Meyer

35

DISCUSSION Trapping and Radio Tagging During Winter Short-tailed hawks do not have a spatial focus for their activities during winter as they do in the nesting season, making them in this respect more difficult to trap than breeding birds. Winter trapping was labor intensive and unpredictable, but we learned that success was highest in the Florida Keys from mid-November to mid-December. A bow net with a starling lure was the most successful trap, and it also was the most portable and fastest to set up. Initially, the VHF transmitters that we used had lightweight wire antennas (twisted steel within a thin plastic coating totaling about 0.7 mm in diameter) attached directly to the base of the transmitter without any means of reducing strain when pulled on by the hawk. Recovered transmitters and field observations suggested that some hawks, especially adults, pulled and twisted the antenna vigorously, which may have caused premature transmitter failure. During the present study, we began using the same model transmitter fitted with a heavier antenna (plastic-coated twisted steel of 1.4-mm diameter) attached with a steel spring that was embedded in the transmitterâ&#x20AC;&#x2122;s epoxy case. This arrangement was much more durable and added negligible weight. The harness design has been well tested on swallow-tailed kites (personal observation). We were confident that the transmitterâ&#x20AC;&#x2122;s attachment did not cause disturbance or discomfort or influence behavior or survival to any meaningful extent. The Teflon harness material, which is strong, nonabrasive, and nonphotosensitive, has been widely used for transmitter harnesses with excellent results. The cotton thread at the closure will eventually rot, allowing both the front and back harness loops to open simultaneously, thus ensuring that the transmitter and harness will fall free of the bird. The harness was quickly attached in the field and could be accurately fitted and adjusted with little difficulty. Finding Nests Our goal for the present study was to find 8 nests over the 2 years. We found 17 nests. Sighting reports from some of the agency staff, particularly FWC, gathered in the course of organized surveys or opportunistic encounters with the species, were especially helpful in identifying possible nest sites. Sighting reports from birders were often interesting, but did not contribute a great deal to directing our nest-searching efforts. Even with information from sighting


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reports, locating short-tailed hawk nests without benefit of tracking radiotagged birds is extremely time consuming and labor intensive. It is no way to build adequate samples for addressing demographic questions or to accrue a substantial catalog of nest-site locations to direct conservation efforts. Nest structures are rarely used from one year to the next, and even though a pair may move only 100â&#x20AC;&#x201C;300 m from a previously used nest tree, finding the new nest requires nearly as much time and effort as it took to locate the first nest in that area. The courtship phase, when hawks frequently perch and vocalize near their nest or the tree in which it will be built, offers a good window of opportunity. Carefully listening for the exchange of calls between mates, moving closer, and then waiting for more vocalization is the best strategy to use to locate the nest because of the limited visual range in closed-canopy swamp forests. Once this period passes, however, and the hawks are incubating eggs, they become extremely inconspicuous, making it necessary to find the actual nest structure with few or no clues from the hawksâ&#x20AC;&#x2122; behavior. After the eggs hatch and adult male feeding rates increase, vocalizations again become helpful for finding the nest. With the high rate of nest failure during the incubation stage, however, a large portion of the potential sample is lost if nests are not found during courtship or early incubation. We were frequently denied access to privately owned lands. Finding and monitoring nests requires many visits to a site, and some private landowners allowed us on their property for only 1 or 2 escorted visits, which were of very limited use. Others completely denied access. Because we were relying on direct visual observations to find nests during the first 3 years of our studies, there was a bias in the types of nest sites we were able to find. We were far less likely to find nests in the interior of large, closed canopy stands because few informative observations would come from such areas, and, even if they did, the probability of finding the nest structure was small. The best way to eliminate this bias is to use radio-tagged birds to find nests. This requires considerable flight time to make the initial breedingseason contact with the marked hawks after they have shifted from their winter to summer range. Although this may seem an expensive alternative to groundsearching for nests, it is actually more cost-effective considering the amount of ground effort required to find a nest and the low probability of success. With an increased flying budget for the last year of the study, we found 7 nests that would never have been found or even suspected by means of opportunistic sightings or systematic visual surveys. The 7 breeding females at these nests, furthermore, represented all the females we had targeted for


SHORT-TAILED HAWK IN FLORIDA—Meyer

37

location that summer based on their age and potential as breeders. These 7 telemetry-located nest territories provided a very different picture of “suitable” nest habitat for short-tailed hawks than that derived from the visually located sites. Preferred nesting habitat is characterized more by larger, denser tracts of forest that allow the nest to be positioned a considerable distance from the forest edge to minimize disturbance or detection. This is an important finding with regard to short-tailed hawk conservation and it deserves further attention. Monitoring Nests We used 32 days for the incubation period and 37 days for the nestling period, a compromise between results reported by Ogden (1988) for 2 south Florida nests and our own observations at various nests during the course of the study. The precise length of the nestling period in this species is unknown. Although short-tailed hawks can branch at approximately 25–30 days, they probably do not leave the immediate vicinity of the nest until 7–14 days later. Our estimates of nesting success and productivity for Florida’s population of short-tailed hawks were very low. Short-tailed hawks are on a par with snail kites in this regard and substantially lower than all other species considered, particularly in comparison with the other 3 Buteo species (Table 12).

Table 12. Comparison of nesting success and productivity between short-tailed hawks and 6 other species of raptors that breed in Florida.

Species Short-tailed hawk (Buteo brachyurus) Snail kite (Rostrhamus sociabilis) Red-shouldered hawk (Buteo lineatus) Red-tailed hawk (Buteo jamaicensis) Broad-winged hawk (Buteo platypterus) Cooper’s hawk (Accipiter cooperii) Swallow-tailed kite (Elanoides forficatus)

Traditional nesting success

Mayfield nesting success

Young fledged per attempt

0.41

0.37

0.64

0.32 0.42 0.30–0.46 0.48 0.68

no data no data 0.23-0.36 no data no data

no data no data 0.44–0.76 no data 1.8

Snyder et al. 1989 Sykes et al. 1995 Bennetts et al. 1988 Sykes 1979 Henny et al. 1973

0.83 no data 0.50–1.0

no data no data no data

no data 1.36 1.5–2.0

Mader 1982 Johnson 1975 Goodrich et al. 1996

0.53–0.85

no data

1.6–2.8

no data

no data

0.9

Rosenfield and Bielefeldt 1993 Meyer 1995

Source This study


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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

Although we cannot explain the ultimate causes of the short-tailed hawk’s low nesting success and productivity in Florida, we did identify the incubation stage as the period when most failures occurred. We also have presented very good evidence that this is a result of infertility, embryo mortality, or hatching failure and not depredation. Once eggs hatched, survival of nestlings and yearlings appeared to be quite good, and predation during these periods was apparently relatively low. We lack sufficient data, particularly on survival, to model Florida’s population demographically. Relative to ecologically similar species, however, there is cause for concern regarding the species’ poor reproductive performance. Apart from the short-tailed hawk’s extremely small population size, this aspect of their demography should be the most important consideration regarding the viability of the Florida population. Given the limitations of VHF radio telemetry in our studies, we were fortunate to obtain some definitive information on age of first breeding. It is encouraging that at least some females in this population of short-tailed hawks attempt to breed at 2 years of age. Many individuals, especially males, may not try to reproduce until 3 or 4 years of age, as in other species of raptors with similar ecology and life history characteristics (Newton 1979, Johnsgard 1990). Our results suggest that the average age of first breeding, at least for female short-tailed hawks, is probably closer to 2 years than to 3 or 4 years. Age of first breeding can have an important influence on lifetime reproductive success, particularly in a species with a small clutch size and low nesting success (Partridge 1989). Nest-site Vegetation Our analyses of nest-site vegetation, limited to measurements within a 0.4ha circular plot centered on the nest tree, indicate that short-tailed hawks selected nest trees that were among the largest and probably oldest trees in the stand. Our measurements, including basal areas and crown closure, support visual assessments that the nest stands were mature with high stem densities and large trees. As stated above, unbiased detections of nests indicate that many, and perhaps most, short-tailed hawks prefer sites well within large, mature stands of mixed species swamp forest. We had no prior reports suggesting breeding activity at any of the areas surrounding the 7 radio-located nests in 2001. When monitoring the status of these nests, the most we ever saw at any point


SHORT-TAILED HAWK IN FLORIDA—Meyer

39

in the approach to the nests was a small portion of the stationary bird on the nest when we came within visual range of the nest tree—and as often as not even this was invisible. We inferred incubation or brooding from the transmitter’s signal. With rare exceptions, neither the male nor female hawk appeared in the air or vocalized as we approached and departed the nest area. Knowing this, and visualizing the setting in which these nests were located, it is easy to imagine a skilled but uninformed observer spending substantial periods of time immediately adjacent to these forests without suspecting or finding a nest and, probably, without even seeing a short-tailed hawk. Finding nests solely by visual observations has undoubtedly biased our, and previous workers’, impressions of where short-tailed hawks nest. It also means that nesting territories of short-tailed hawks probably are being lost as mature swamp forest is destroyed, with no one ever even knowing that the site had been occupied, perhaps for many years, by breeding birds. Trapping and Radio Tagging Adults and Young at Nests Trapping and radio tagging adults and nestlings in the summer gradually became successful enough to be useful as a techniques for studying shorttailed hawks. Trapping these birds at nests will always be labor intensive and highly unpredictable. We learned enough, however, and were able to sufficiently refine the trapping techniques to conclude that trapping at nests is feasible, if costly. We had learned in the previous study that nestlings had to be captured earlier than originally thought—23 days rather than 28 days—because when older, they moved out of the nest and beyond reach or flew from the tree as the climber approached. Most young had achieved average adult weight by the time they were captured at 23 days, indicating that the transmitter harness could be accurately fitted, something that is difficult to do when nestlings have not yet reached adult weight. It is essential to climb at exactly the correct point in the birds’ development so that they will not fledge prematurely, yet will be large enough to be fitted safely with the transmitter harness. The limiting factors are access, condition of the tree, location of the nest within the tree, and being able to monitor nests frequently enough to judge the age of the nestlings and predict the best dates for climbing. For breeding adults, 2 types of traps showed the most promise, although both required much effort and had variable success rates. A mist net and live owl lure placed near the nest probably was the better method, but the hawks frequently bounced out or escaped from the net. A bow net with a prey-bird lure set away from the nest was successful in 1 of 2 attempts, but we did not


40

FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

try it enough during the summer to judge its effectiveness. This method was most successful during winter when the hawkâ&#x20AC;&#x2122;s movements were not tied to a specific site. Although we did not meet the proposed objectives for trapping adult and young hawks at nests, our winter captures totaled more than double the maximum number proposed. In the second year of the study, we shifted our resources (i.e., budgeted funds for personnel time and field logistics) more toward winter trapping than summer trapping because more birds could be captured in winter per unit of cost. The larger number of winter-caught birds also gave us an advantage in finding nests the following spring once we expanded our flying time. As a result, we found more than twice as many nests (17) as proposed (8). Summer Activity Ranges Our sample size of radio-tagged breeding adults was very small, and available resources did not permit us to monitor nesting hawks throughout the breeding season at regular intervals. Our efforts were compromised mainly by the demands of finding nests, which we continued to work at as long as young remained in the known nests. We would have preferred to collect data, including more locations per bird, with a more evenly stratified sampling protocol. Finding short-tailed hawk nests is extremely labor intensive and unpredictable. Had we spent less time finding nests, we would have had more time to track birds. We would have had fewer birds to track, however, further limiting our small sample. Our data support speculation that the activity ranges of short-tailed hawks rearing young are smaller than what might be predicted based on the speciesâ&#x20AC;&#x2122; size and foraging ecology (they eat birds, which are difficult to capture) and on activity-range data for ecologically similar species (Newton 1979). The hawksâ&#x20AC;&#x2122; locations were tightly clustered around the immediate nest area over the entire period for which we obtained radio fixes, regardless of the stage of the nesting cycle. In addition, hawks with breeding territories arrived early in the season (mid-February), consistently occupied their nesting ranges, and remained in the immediate area of the nest well into October, long after their young became independent. This suggests some advantages for defending, at least by occupancy, resources associated with the nesting site. The summer ranges of the 3 1- and 2-year-old non-breeding hawks (#4.464, #4.585, and #4.500) were large relative to those of breeding hawks


SHORT-TAILED HAWK IN FLORIDAâ&#x20AC;&#x201D;Meyer

41

and were comparable to these birdsâ&#x20AC;&#x2122; respective winter ranges. It is interesting that the summer ranges of the 2 birds for which we knew their natal territory (#4.464 and #4.500) included their natal territories. Both were males, and their transmitters expired before their third summer, so we do not know if they attempted to nest in that area. The summer range of the third male bird (#4.585), whose natal territory was not known, included 5 of the 14 nest territories we monitored. The summer range and movements of male Sat 4 were similar to those of the pre-breeding VHF birds described above. One of these pre-breeding VHF birds, female #5.829, was still being tracked the next summer when she successfully nested within her nonbreeding range of the previous summer. Sat 4 displayed the same behavior, nesting in 2002 within his 2001 nonbreeding range. The lack of significant selection for, or avoidance of, specific cover types by Sat 4 during summer 2001 is not easily explained, but it may be due, at least in part, to his use of urban fringe areas adjacent to patches of natural forest that he regularly used. Short-tailed hawks often prey on birds, particularly exotic doves, in suburban areas of Broward, Dade, and Monroe counties in southern Florida (Robertson and Woolfenden 1992). In addition, one might infer that a bird using habitats in proportion to their availability is doing so because it occupies a high-quality, efficient activity range. In the case of Sat 4, the fact that he chose to nest in 2002 in a concentrated portion of the area he occupied the previous summer as a non-breeder (the season on which the habitat analysis was based) supports this inference. Perhaps even in high-quality habitat, non-breeding short-tailed hawks maintain much larger activity ranges than would be necessary to support themselves or a nesting effort so that they can look for breeding opportunities, identify potential mates, and begin making choices about potential nest sites. Winter Activity Ranges Winter activity ranges of VHF birds were large, although core areas were evident even with relatively small samples of locations. There also was a strong indication that the birds shifted the focus of their activity at least once during the winter. Tracking locations of marked birds, visual observations in the region, and results of our trapping efforts all indicate that short-tailed hawks arriving in southern Florida from late September through October concentrate at the southern end of the peninsula, mainly in the coastal habitats from Pavilion Key southeastward through Cape Sable and eastward along the


42

FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

northern shore of Florida Bay. By mid-November, however, they radiate both northward and into the Florida Keys. These shifts in activity range may be related to seasonal changes in abundance of birds on which the hawks are preying. The predictable concentration during a specific time of year offers opportunities for long-term monitoring. The clearest indication of a dichotomous winter distribution comes from the satellite data for Sat 4, for which we have the most information. In 2 successive winters, 2000 and 2001, this bird first occupied a well-defined range centered roughly on Flamingo at the southern tip of the peninsula (Fig. 8). In mid-December of both years, he moved northward to Big Cypress Swamp, centering his activity on Big Cypress Indian Reservation (Fig. 8). In 2000, Sat 4 began this northward move on 11 December; in 2001, he did so on 10 December. As Fig. 8 indicates, both the early and late ranges were extremely similar in location and size between years. Satellite telemetry and GIS analyses of wintering habitat for Sat 4 revealed a clear association with the same types of wetland forests (mixed hardwood swamp forest, mangrove swamp, and cypressâ&#x20AC;&#x201C;gum swamp) consistently occupied by breeding and nonbreeding birds during the summer. Agriculture and freshwater marsh were the habitats least likely to be used by the wintering hawk, and other treeless natural cover types (non-vegetated wetlands, freshwater wetlands, and open water) also were used relatively little, perhaps because they were unforested. Satellite-derived data and habitat analysis for Sat 4, although for only 1 bird, are the best available characterization of year-round habitat selection and area requirements of short-tailed hawks in Florida. Combined with descriptions of nest-site and landscape vegetation, these results highlight the importance of forested wetlands to short-tailed hawks. Survival Our samples for the analysis of juvenile survival were relatively small, but probably would have been adequate to draw some conclusion about survivorship if the study was longer. We did not believe that our study design would support a meaningful analysis of adult survival and did not propose doing so. Nonetheless, if one cautiously considers our results, it appears that survival of fledgling and first-year short-tailed hawks was relatively high during the course of our study, suggesting that survival in the first 6 months is in the range of 50â&#x20AC;&#x201C;80%. It is likely, furthermore, that survival might improve during the remainder of the first year. In contrast, swallow-tailed kites in


SHORT-TAILED HAWK IN FLORIDA—Meyer

43

Florida appear to have first-year survival of 15–20% (K. Meyer, unpublished data), and first year survival <50% is typical among raptors (Newton 1979, Johnsgard 1990). The poor nest success that we observed resulted from failures mainly during the incubation stage. Once hatching is achieved, however, it may be that the probability of attaining breeding age is good for Florida’s short-tailed hawks compared with other species of raptors. The very short migration distance, entirely overland, may be a contributing factor. Another consideration may be the species’ very small population size in Florida. If this is the result of conditions other than habitat limitations, then intraspecific competition among adults and pressure on pre-breeding aged birds to disperse may be relatively low. In both cases, mortality could be reduced as a result. Color Morph and Sex Ratios There is some indication from our small sample that the ratio of dark:light birds may increase with age for both sexes (Table 9). These results suggest that changes in color-morph ratios may be worth additional investigation. One possible explanation is differential juvenile mortality between morphs. The suggestion that there is no assortative mating with regard to color morphs (Table 10) is encouraging in that breeding effort in this very small population probably is not restricted by the availability of mates of a specific morph. No conclusions can be drawn about the female-biased sex ratio among adults (5 males:8 females, Table 11) because it is based on winter-trapped birds. For whatever reason(s), females may be more trappable. It is also possible that there is some segregation by sex on the winter range and females are more prevalent in the Florida Keys, where most of the captures occurred. We do not have sufficient radio-location data for birds wintering in the Keys to determine whether females are more likely to winter there. No such potential biases exist, however, for birds sexed as nestlings. Assuming there were no sex-specific differences in mortality prior to the time when we removed nestlings for radio tagging and collecting blood for sexing (about 24 days of age), our sample should reflect the primary sex ratio. The limitation in our sample is its size (n = 8), not its potential biases. Nonetheless, the ratio of 7 males:1 female (Table 11) suggests that further investigation is warranted. Skewed sex ratios, particularly in non-communally nesting birds, have been explained in some cases as a response to pronounced


44

FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT

changes in the quality of the nesting environment. For example, if conditions are poor locally, the dispersing sex may be favored during embryo production or even by differential parental care of nestlings; if conditions are unusually favorable, the non-dispersing sex may be favored (Sheldon 1998, Paxton et al. 2002). It is unfortunate that the logistics of finding nests, and sampling young from those that eventually succeed, limit our ability to achieve an adequate sample size in any reasonable period of time. In our case, it took 4 years to sex 8 nestlings.


SHORT-TAILED HAWK IN FLORIDA—Meyer

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IMPLICATIONS FOR MANAGEMENT AND CONSERVATION Several key conclusions of our studies should be of interest and concern to wildlife managers and conservationists. We are confident that these specific points are well supported by our observations over the last 4 years and by our subsequent analyses. Population Size By any standards, the U.S. population of short-tailed hawks, confined to Florida, is very small and extremely vulnerable. The upper limit of the current estimate is 500 individuals (Ogden 1988, Millsap et al. 1989), or no more than 200 breeding pairs (allowing for nonbreeders and young of the year). We conclude from our studies that the Florida population most likely includes less than 200 pairs attempting to nest in any year. This very small population size makes Florida’s short-tailed hawks highly vulnerable to every factor that can threaten wildlife: catastrophic climate conditions, disease, loss of suitable habitat, human persecution, and genetic anomalies. One additional aspect of demography, sex ratio, should be examined further due to the suggestion from our work that females may be underrepresented among nestlings. Low Nesting Success and Productivity There is no question that the short-tailed hawk’s reproductive performance ranks at the lowest end of the scale in comparison with similar species. Low nesting success in our studies resulted almost entirely from nest failures during the egg stage. Predation and abandonment could not be implicated as factors. Rather, the proximate causes of nesting failure apparently were infertile eggs, embryo mortality, or hatching failure. An encouraging conclusion is that fledging success for young that hatch is very good and first-year survival appears to be high. Given that nesting success is so low, and that the period associated with the failures is so evident, further investigation of the more specific, ultimate causes of nest failures should be attempted as soon as possible. Nesting Habitat In all cases studied, short-tailed hawks in Florida nested in dense, mature stands of wetland forest. Nests that were found by radio telemetry, a method unbiased by physical access and observability of the nest stand, were in the interior of large stands of dense, mature wetland forest. Such large tracts are becoming increasingly rare on Florida’s privately owned lands, where most short-tailed hawks probably nest. Improving harvest technology, diversifying


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timber markets, and extended droughts will combine to increase the rate at which these valuable forests are destroyed. Additional Management Needs Development and implementation of a long-term monitoring plan for short-tailed hawks in Florida should be given high priority. I have written a draft plan for the North American Raptor Monitoring Scheme (NARMS) initiative of the U.S. Geological Survey. The goal of NARMS is to develop statistically adequate and affordable monitoring protocols that will detect specific decreases in population size within set time periods for the continentâ&#x20AC;&#x2122;s raptor species Federal listing of the short-tailed hawk is rarely discussed. Perhaps this is because the population-trend requirement for securing Endangered or Threatened status, whereby present or imminent population declines must be rigorously demonstrated, has been so difficult to pursue for this species. This should serve as additional justification for implementing a sound monitoring plan. We should remember, however, that in the case of the very small and minimally productive Florida population of short-tailed hawks, it could be impossible or prohibitively expensive to reverse a declining trend even if action is taken as soon as the decline is detected.


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LITERATURE CITED Bennetts, R. E., M. W. Collopy, and S. R. Beissinger. 1988. Nesting ecology of the snail kite in Water Conservation Area 3A. Technical Report 31. Florida Cooperative Fish and Wildlife Research Unit, University of Florida, Gainesville, Florida, USA. Brandt, H. W. 1924. The nesting of the short-tailed hawk. Auk 41:59–64. Brown, L., and D. Amadon. 1968. Eagles, hawks and falcons of the world. McGraw-Hill, New York, New York, USA. Earth Systems Research Institute. 1996. ArcView GIS. Earth Systems Research Institute, Redlands, California, USA. Goodrich, L. J., S. C. Crocoll, and S. E. Senner. 1996. Broad-winged hawk. In A. Poole and F. Gill, editors. Birds of North America, No. 218. Academy of Natural Sciences, Philadelphia, Pennsylvania, and American Ornithologists’ Union, Washington, D.C., USA. Henny, C. J., F. C. Schmid, E. M. Martin, and L. L. Hood. 1973. Territorial behavior, pesticides, and the population ecology of red-shouldered hawks in central Maryland, 1943–1971. Ecology 54:545–554. Hensler, G. L. 1985. Estimation and comparison of functions of daily nest survival probabilities using the Mayfield method. Pages 289–301 in B. J. T. Morgan and P. M. North, editors. Statistics in ornithology. SpringerVerlag, New York, New York, USA. _____, and J. D. Nichols. 1981. The Mayfield method of estimating nesting success: a model, estimators, and simulation results. Wilson Bulletin 93:42–53. Hooge, P. N., and B. Eichenlaub. 1997. Animal movement extension to ArcView, version 1.1. Alaska Biological Science Center, U.S. Geological Survey, Anchorage, Alaska, USA. Jennrich, R. I., and F. B. Turner. 1969. Measurement of non-circular home range. Journal of Theoretical Biology 22:227–237. Johnsgard, P. A. 1990. Hawks, eagles, and falcons of North America. Smithsonian Institution Press, Washington, D.C., USA.


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Johnson, S. J. 1975. Productivity of the red-tailed hawk in southwestern Montana. Auk 92:732–736. Kaplan, E. L., and P. Meier. 1958. Nonparametric estimation from incomplete observations. Journal of American Statistics Association 53:457–481. Mader, W. J. 1982. Ecology and breeding habits of the Savanna hawk in the llanos of Venezuela. Condor 84:261–271. Mayfield, H. 1961. Nesting success calculated from exposure. Wilson Bulletin 73:255–261. _____. 1975. Suggestions for calculating nest success. Wilson Bulletin 87:456–466. Meyer, K. D. 1995. Swallow-tailed kite. In A. Poole and F. Gill, editors. The birds of North America, No. 138. Academy of Natural Sciences, Philadelphia, Pennsylvania, and American Ornithologists’ Union, Washington, D.C., USA. Miller, K. E., and K. D. Meyer. 2002. The short-tailed hawk. In A. Poole and F. Gill, editors. The birds of North America, No. 674. Academy of Natural Sciences, Philadelphia, Pennsylvania, and American Ornithologists’ Union, Washington, D.C., USA. Millsap, B. A., J. A. Gore, D. E. Runde, and S. I. Cerulean. 1990. Setting priorities for the conservation of fish and wildlife species in Florida. Wildlife Monograph Number 111. Wildlife Society, Bethesda, Maryland, USA. _____, M. Robson, and D. E. Runde. 1989. Short-tailed hawk surveys. Annual performance report. Florida Game and Fresh Water Fish Commission, Tallahassee, Florida, USA. _____, _____, and B. R. Toland. 1996. Short-tailed hawk. Pages 315–322 in J. A. Rodgers Jr., H. W. Kale II, and H. T. Smith, editors. Rare and endangered biota of Florida. Volume 5: Birds. University Press of Florida, Gainesville, Florida, USA. Mohr, C. O. 1947. Table of equivalent populations of North American small mammals. American Midland Naturalist 37:223–249.


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Moore, J. C., L. A. Stimson, and W. B. Robertson Jr. 1953. Observations of the short-tailed hawk in Florida. Auk 70:470–478. Newton, I. 1979. Population ecology of raptors. Buteo Books, Vermillion, South Dakota, USA. Nicholson, D. J. 1951. Notes on the very rare short-tailed hawk. Florida Naturalist 24:33–34. Norris-Caneda, K. H., and J. D. Elliott Jr. 1998. Sex identification in raptors using PCR. Journal of Raptor Research 32:278–280. Ogden, J. C. 1974. The short-tailed hawk in Florida. I: Migration, habitat, hunting techniques, and food habits. Auk 91:95–110. _____. 1988. Short-tailed hawk. Pages 34–47 in R. S. Palmer, editor. Handbook of North American birds. Volume 5. Yale University Press, New Haven, Connecticut, USA. Partridge, L. 1989. Lifetime reproductive success and life history evolution. Pages 421–440 in I. Newton, editor. Lifetime reproduction in birds. Academic Press, New York, New York, USA. Paxton, E., M. K. Sogge, T. D. McCarthey, and P. Keim. 2002. Nestling sex ratio in the southwestern willow flycatcher. Condor 104:877–881. Pollock, K. H., S. R. Winterstein, C. M. Bunck, and P. D. Curtis. 1989. Survival analysis in telemetry studies: the staggered entry design. Journal of Wildlife Management 53:7–15. Robertson, W. B., Jr., and G. E. Woolfenden. 1992. Florida bird species. Special publication number 6. Florida Ornithological Society, Gainesville, Florida, USA. Rosenfield, R. N., and J. Bielefeldt. 1993. Cooper’s hawk. In A. Poole and F. Gill, editors. The birds of North America, No. 75. Academy of Natural Sciences, Philadelphia, Pennsylvania, and American Ornithologists’ Union, Washington, D.C., USA. Service Argos. 1996. User’s manual. Service Argos, Landover, Maryland, USA.


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Sheldon, B. C. 80:397–402.

1998.

Recent studies of avian sex ratios.

Heredity

Snyder, N. F. R., S. R. Beissinger, and R. Chandler. 1989. Reproduction and demography of the Florida Everglade (snail) kite. Condor 91:300–316. Sprunt, A., Jr. 1939. Short-tailed hawk in Florida. Auk 56:330–331. Stevenson, H. M., and B. H. Anderson. 1994. The birdlife of Florida. University Press of Florida, Gainesville, Florida, USA. Sykes, P. W., Jr. 1979. Status of the Everglade kite in Florida—1968–1978. Wilson Bulletin 91:495–511. _____, J. A. Rodgers Jr., and R. E. Bennetts. 1995. Snail kite. In A. Poole and F. Gill, editors. The birds of North America, No. 171. Academy of Natural Sciences, Philadelphia, Pennsylvania, and American Ornithologists’ Union, Washington, D.C., USA. Tordoff, H. B. 1954. An automatic live-trap for raptorial birds. Journal of Wildlife Management 18:281–284. White, G. C., and R. A. Garrott. 1990. Analysis of wildlife radio-tracking data. Academic Press, New York, New York, USA.


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Biology and Conservation Needs of the Short-tailed Hawk in Florida  

Meyer

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