Page 26

not limiting, and that the reason pileated woodpeckers establish and defend large territories in Alberta foothills forests (Chapter 3) is to secure winter food resources (McClelland 1979), which I believe were probably a limiting factor in my study area. I observed selection at the territory and stand scales consistent with the view that pileated woodpeckers are primarily associated with mature/old forest (Conner 1980, Bull and Holthausen 1993, McClelland and McClelland 1999). However, I also documented substantial use of all available habitat categories, which indicates that pileated woodpeckers are not mature-forest obligates and further that they are flexible in habitat use and selection at the territory and stand scales (Hoyt 1957, Mellen et al. 1992, Rolstad et al. 1998). Pileated woodpeckers selected habitats that had the most preferred foraging substrates, but they exploited foraging substrates wherever they were available, irrespective of surrounding habitat. The exception to this was avoidance of open habitats with no standing trees that could be used to escape predators (Lima, 1993). I conclude that habitat selection at the territory and stand scales was primarily a function of foraging substrate availability and secondarily a function of predator avoidance. Selection was identified at all scales but was most significant at the foraging substrate scale. Pileated woodpeckers foraged on substrates containing accessible wood-dwelling ants and other arthropods. In general these were large wood substrates that were not healthy living trees. Large substrates were probably selected because carpenter ants and other prey items were more abundant in large substrates (Sanders 1964, 1970; Bull et al. 1986; Renken and Wiggers 1989). In winter, pileated woodpeckers foraged at the base of injured living trees and hard snags and stubs. In summer, the breadth of substrates used expanded to include soft stubs, logs, and stumps, and living trees with injured or dead tops. Living balsam poplar was selected for summer surface foraging. This species has deeply furrowed and often loose bark and frequent dead tops, which may be associated with increased arthropod abundance (Jackson 1979, Conner et al. 1994). Cavity Trees Cavity trees were found more than expected in tall, medium-density, mixed and deciduous stands at low elevations. Other studies also found most trees in closed canopy forests with more potential cavity trees and higher basal area than available habitat (Conner et al. 1975, Bull 1987, McClelland 1977, Bull et al. 1992a, Bull and Holthausen 1993). Cavity trees were found in all forest and stand types, including open areas, indicating that pileated woodpeckers showed considerable flexibility in cavity tree selection at the territory and stand scales. Cavity trees located in young, open, stands have been previously reported (Conner 1973, McClelland 1977, Millar 1992, Naylor et al. 1997). Use of these habitats, including young forest and urban areas with large remnant trees (Bull 1987, Bull and Jackson 1995), indicates that availability of potential cavity trees is probably the most important selection factor at the territory and stand scales (Kirk and Naylor 1996). Within stands, pileated woodpeckers selected sites with high basal area and more potential cavity trees, but there were fewer trees overall and cavity tree sites were more open than random sites. Selection for more potential cavity trees is consistent with previous reports (Bull 1987), but selection for open sites has not been previously reported for pileated woodpeckers. In Sweden, the black woodpecker (Dryocopus martius) also preferred open sites around cavity trees, possibly because open sites were less vulnerable to predators (Rolstad et al. in review). Failed cavity-user nests in an Arizona study were more concealed by foliage and were closer to conifers than successful nests (Li and Martin 1991). Pileated woodpeckers often perched in a nearby tree in an alert position before approaching cavity trees. Potential predators such as the northern goshawk (Accipiter gentilis) and American marten (Martes americana) would be easier to detect when there are open areas around cavity entrances. In addition to reducing predation risk, nest trees in open areas may also reduce competition for cavities (Short 1979). Pileated woodpeckers excavate cavities in large trees, often the largest available (Conner et al. 1975, McClelland 1979, Peck and James 1983, Bull 1987). Large trees are less likely to break at the cavity location (Harris 1983, Harestad and Keisker 1989), and they are more likely to have stem decay (Basham 1958, Hiratsuka et al. 1990). Cavity tree size in this study was comparable to that of similar forest regions (Campbell et al. 1990, Harestad and Keisker 1989, Millar 1992), 16

Pwp 2001 04 rpt phdthesis pileatedwoodpeckerhabitatecologyinabfoothills