Warblers from 2018-19 in Northeastern North Carolina

Summers with Golden Swamp Warblers

Aspects of the Breeding Biology of Prothonotary Warblers from 2018-2019 in Northeastern North Carolina

Skadi Kylander

ABSTRACT

Every summer, southern swamps east of the Mississippi River host the only cavity-nesting species of warbler to breed in eastern North America. Prothonotary Warblers (Protonotaria citrea) rely on oldgrowth swamps in this area for breeding and to raise their young. Population declines in recent decades have sparked interest in understanding Prothonotary Warbler breeding biology. In 2018 and 2019, nest box occupancy, nest success, nestling growth rate, types of prey brought to nestlings, and standard measurements of adults were recorded in two eastern North Carolina habitats: the Chowan University Meherrin River Field Site (MRFS) with riparian and bottomland hardwood swamps, and a maritime forest containing depressional wetlands in the Kitty Hawk Woods Coastal Reserve (KHW). Six nest boxes were installed at each site for the 2018 breeding season. This was increased to 19 boxes at MRFS and 18 boxes at KHW for the 2019 season. Observations from the two breeding seasons yielded a total of 15 occupied nest boxes (11 at MRFS; 4 at KHW). While the percent of boxes occupied at MRFS (44%) appeared significantly higher (p = 0.04) than at KHW (17%), nest success at MRFS was significantly lower than at KHW (50% and 96% respectively, p = 0.0001). Likewise, comparison of mean nestling growth rates (MRFS = 0.9 g/ nestling/day; KHW = 1.2 g/nestling/day) approached significance (p = 0.06), with MRFS growth rates appearing lower. Preliminary analysis of nest provisioning suggests prey types and sizes differed between the two sites. Data was collected for a total of 14 adults between the two breeding seasons.

INTRODUCTION

Every summer, North America’s southern swamps east of the Mississippi River play host to the only cavity-nesting species of warbler to visit that part of the continent. These unique visitors are Prothonotary Warblers (Protonotaria citrea). Prothonotary Warblers are Nearctic-Neotropical songbirds (Petit, 1999) that winter in Central and South America (Tonra et al., 2019). They travel to North America for the duration of their breeding season, arriving in the spring and settling into old-growth swamps to breed and to raise their young. In recent decades, Prothonotary Warbler populations have declined, prompting their placement on several watch lists (Rosenberg et al., 2016; USFWS, 2008) and increasing interest in understanding their life cycles to develop better conservation efforts. Gaining an understanding of an organism’s full life cycle has been shown to aid in effective conservation (Marra et al., 2015). Though these birds spend a significant portion of the year in Central and South America (Tonra et al., 2019), northeastern North Carolina has been shown to contain one of the highest breeding densities of Prothonotary Warblers in North America (Petit, 1999), making it a promising area to use to study the breeding portion of their life cycle.

A pilot study on Prothonotary Warblers nesting in northeastern North Carolina was started by Dr. Heather McGuire and Dr. James Dame from the Department of Biology at Chowan University in Murfreesboro, NC in 2018. As of 2019, the purpose of this study is to document and compare differences

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in breeding biology of these birds in two different habitats. These sites were chosen based on where Drs. McGuire and Dame had heard the sweet-sweet-sweet song of Prothonotary Warblers during prior field seasons. This project was expanded for the 2019 breeding season. I became involved with this project around this time.

METHODS

For the 2018 and 2019 seasons, two sites were selected: Chowan University’s Meherrin River Field Site in Hertford County, NC (MRFS) and the Kitty Hawk Woods Coastal Reserve in Dare County, NC (KHW). MRFS contains bottomland hardwood and riparian wetlands, where an abundance of Water Gum (Nyssa aquatica), Bald Cypress (Taxodium distichum), Sycamore (Platanus occidentalis), Sweetgum (Liquidambar styraciflua), and other trees grow, as well as numerous species of herbaceous vegetation. KHW is a maritime forest with depressional wetlands that is home to a similar variety of trees but has a different community herbaceous vegetation. Though there are similarities when looking at the vegetation, personal observations and preliminary data collected from in situ water level loggers suggest each site experiences distinctly different hydroperiods (H. McGuire and J. Dame, personal communication, 2019), or differences in how water is present at each site.

Pine nest boxes were installed at each study site. These boxes were constructed from 1” x 4” pine lumber according to a plan from Audubon Louisiana with 1¼’’ holes to allow access for the birds. The inner dimensions of the boxes are approximately 7.25” x 4” x 2”. These nest boxes have a side door, held shut with a nail, to allow access inside the box.

To install the boxes at the sites, lengths of metal conduit were driven into the ground in areas either over water or on land adjacent to water. The boxes were attached to the conduit with bolts. Under the boxes, PVC predator guards were also attached with bolts (Figure 1). If the box was installed over land, the box was turned such that the hole faced the water, and, if the box was installed over water, the box was turned such that the hole faced land, as recommended by The Cornell Lab of Ornithology (2020).

Figure 1: A nest box installed at KHW

Prior to the 2018 breeding season, six boxes were installed at each site. Prior to the 2019 breeding season, an additional 13 boxes were installed at MRFS and an additional 12 boxes were installed at KHW. After the boxes were installed, they were left alone until observations began in early May of both years.

Observations of Prothonotary Warbler nests followed protocols established by Bulluck et al. (2016). Because the sample sizes each year were small, we combined our data for the two breeding seasons for statistical analyses.

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The number of nest boxes occupied, eggs laid, nestlings hatched, and nestlings fledged at each occupied nest box were recorded. In our calculations, an occupied nest box was defined as a nest box with both nesting material and eggs. Nest box occupancy was then calculated as the number of occupied nest boxes divided by the total number of nest boxes available. Calculations on success were based on success being defined as the number of eggs laid that produced fledglings capable of flight. Two-tailed Z-tests were used to determine the significance of the difference in nest occupancy and of the difference in nest success between MRFS and KHW (H0: MRFS = KHW, α = 0.05).

When possible, nestlings were weighed twice during the nestling phase to estimate an average growth rate (calculated as g/nestling/day). Our nestlings were weighed once when they were 3-5 days old and again when they were 7-9 days old. Nestling growth rate is approximately linear at this age (Podlesak & Blem, 2002), which allows us to make an estimation of the average growth rate. A two-tailed t-test was used to determine if the difference in average growth rates between the two sites was significant (H0: MRFS = KHW, α = 0.05).

Nest boxes were also observed while nestlings were present to record the frequency of adult visits and the amount and types of food items delivered to nestlings by adults. Video was taken during the majority of these provisioning observations using a Panasonic Lumix DMC-ZS20, a Panasonic Lumix DC-ZS70K, or a Canon EOS Rebel T5 Digital SLR camera equipped with an EF 75-300mm telephoto zoom lens.

Adults at active nests were captured with either a hand net or with a mist net, when possible. Captured adults were banded (USFWS aluminum band; banding permit held by H. McGuire) or had their band numbers recorded, if they were already banded. Standard measurements were also recorded. These included age; sex; length of the tarsus, culmen, wing chord, and tail; degree of cloacal protuberance and brood patch presence; and weight.

RESULTS

Nest boxes were occupied during both breeding seasons (Table 1). The two breeding seasons combined yielded 44% nest box occupancy by Prothonotary Warblers at MRFS and 17% at KHW. This difference is significant (Z = 2.0753, p = 0.04), with a greater number of nest boxes being occupied at MRFS (11 of the possible 25 boxes) than at KHW (4 of the possible 24 boxes).

Table 1: Nest boxes installed and occupied (2018; 2019)

Site Boxes installed Boxes occupied

MRFS 6; 19 2; 9

KHW 6; 18 2; 2

In 2019, 11% of the installed boxes at MRFS were occupied by wasps, while 67% of the boxes at KHW were occupied by wasps. Wasp occupancy was low at both sites during the 2018 breeding season. We

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noted three nest boxes being occupied by Carolina Chickadees (Poecile carolinensis), one at MRFS in 2018 and one at each site in 2019. Nest parasitism by Brown-headed Cowbirds (Molothrus ater) was rare. We only recorded nest parasitism at one nest at MRFS, and it occurred after the access door of the box had fallen open. This nest was abandoned shortly after the parasitism occurred.

Of the boxes installed at MRFS during the 2018 and 2019 breeding seasons, 69% of the eggs laid hatched (25 of 36), and 50% (18 of 36) of the eggs laid produced fledglings capable of flight. At KHW, 96% (24 of 25) of the eggs laid hatched, and 96% (24 of 25) of eggs laid produced fledglings between the two breeding seasons (Table 2). A two-tailed Z-test suggests this difference was highly significant (Z = 3.8154, p = 0.0001), with nests at KHW having greater success than those at MRFS.

Table 2: Eggs laid, hatched, and producing fledglings (2018; 2019)

Site Eggs laid Hatched Fledged

MRFS 36 25 18

KHW 25 24 24

The two breeding seasons yielded a total of seven nests where nestlings were able to be weighed twice to determine their growth rates. At MRFS, nestlings had a mean growth rate of 0.9 g/nestling/day (SD = 0.23). At KHW, nestlings had a mean growth rate of 1.2 g/nestling/day (SD = 0.06). This difference approached significance (t(5) = 2.389, p = 0.06).

Nest provisioning observations, or observations made to attempt to identify the prey brought to nestlings by the adults, were completed during both breeding seasons. Field observations and preliminary review of video recordings suggest the majority of prey delivered to nestlings at the MRFS was fairly large, while prey at KHW was generally smaller (Figure 2).

Data was recorded for a total of 14 adults between the two breeding seasons (8 at MRFS; 6 at KHW). This data has not yet been analyzed, however.

DISCUSSION

Figure 2: Examples of prey brought to nestlings in 2019 (1. MRFS, 2. KHW, 3. MRFS, 4. MRFS)

The nest box occupancy we observed is similar to those reported in other studies (Blem & Blem, 1991; Siverly, 2015). Additionally, when the occupancy rates from our two sites are combined (giving a combined occupancy rate of 31% (15 of 49) between the two seasons), the rate is similar to those observed in larger studies (e.g., Blem & Blem, 1991). One of the major reasons we suspect occupancy was lower at KHW was the presence of wasps in the nest boxes. Of the 18 available nest boxes at KHW in 2019, 67% (12 of 18) of

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the boxes were occupied by wasps, compared with 11% (2 of 19) at MRFS. Blem and Blem (1991) found approximately 34% of their nest boxes were occupied by wasps and concluded that their presence deterred Prothonotary Warblers from nesting in those boxes. We believe wasps may have had a similar effect on our boxes. Though wasp nests were removed when discovered, all but one of our nest boxes that were occupied by wasps were not occupied by Prothonotary Warblers.

Nest success at MRFS is comparable to that reported by others (Podlesak & Blem, 2001; Siverly, 2015). Success at KHW appears much higher than what has been observed in other locations. However, our small sample sizes make us unsure if the differences we have observed between MRFS and KHW are due to these small samples, to seasonal variability (as appears to have been the case with Slevin et al. (2018)), to some inherent characteristic of the sites, or to other factors.

We believe flooding and predation may have affected nest success at MRFS. Nest failure has been connected with flooding in other studies, with inundation being the major direct cause of that failure (Siverly, 2015). Though our boxes were not covered with water, the fluctuations in water level in the Meherrin River led to the partial submergence of many of the predator guards of boxes installed over water, which may have left those nests vulnerable to predation. Other studies have found nests predated by mice (Peromyscus gossypinus and P. leucopus) and raccoons (Procyon lotor), among others (Blem & Blem, 1991). Though we did not directly observe any predation, potential predators, such as raccoons, have been noted as being present in the area.

The difference between nestling growth rates between our two sites was not significant, but it did approach significance (p = 0.06), with nestlings in KHW tending to have higher growth rates than those at MRFS. The growth rates we observed at KHW are similar to those reported by Dodson et al. (2016), who found significant differences in growth rates at two sites where nestlings had different diets. They observed higher growth rates in nestlings fed aquatic prey, while those fed terrestrial prey tended to have lower growth rates. At our two study sites, nestling provisioning seemed to differ in the size of prey and number of prey items delivered at one time rather than if prey was aquatic or terrestrial. Though our observations appear to differ from those of Dodson et al. (2016), it does raise the question as to the potential effects of prey on nestling growth.

We have not yet had an opportunity to analyze the adult data. However, banding data shows 2 of the 9 adults banded in 2018 were recaptured in 2018 (1 female from MRFS and 1 male from KHW). These adults selected nesting boxes in relatively the same area in 2019 as they had used in 2018 (i.e., they either nested in the same box or a nearby box), suggesting adults may return to the same breeding area each year (also noted in Petit, 1999). An additional 5 adults were captured in the 2019 breeding season, for a total of 14 adults between the two breeding seasons.

CONCLUSION

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these two different habitats. Nest box occupancy at MRFS appeared higher than at KHW. This may have been influenced by the high presence of wasps in nest boxes at KHW. Despite the higher occupancy, nest success was significantly lower at MRFS, which may be due to the greater variability of the hydroperiod potentially making nest boxes more vulnerable to predation. It also appeared that there may be a difference in nestling growth rates between these two sites, with higher growth rates observed in KHW. This difference may be related to prey availability. Additionally, though the majority of the adult data has not yet been analyzed, banding data revealed that 2 of our adult birds in 2019 had been present in this area in 2018, suggesting these birds may have nesting areas to which they return year after year.

Future plans include installing additional nest boxes, with the hope of increasing our sample size. We plan to continue documenting nestling provisioning, attempt a systematic documentation of what prey is available at each study site, and begin placing wildlife cameras at active boxes to try to identify potential reasons for nest failure.

ACKNOWLEDGMENTS

A very special thank you to Drs. McGuire and Dame for inviting me to be a part of this project, and for all of their remarkable guidance and support. Thank you, as well, to the Town of Murfreesboro and the North Carolina Coastal Reserve System for allowing us access to the Meherrin River Field Site and Kitty Hawk Woods for this research, and to the Chowan University Ecology Field Camp students for their assistance with nest box installation and preliminary observations. Financial support that helped me work on this project in the 2019 breeding season came from an undergraduate research stipend awarded by the North Carolina Independent Colleges and Universities (NCICU) and from a research grant awarded by the Carolina Bird Club, Inc.

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