Page 75

5. GENERAL DISCUSSION AND CONCLUSIONS PILEATED WOODPECKER HABITAT ECOLOGY IN THE ALBERTA FOOTHILLS Pileated woodpeckers (Dryocopus pileateus) are specialist predators of wood-dwelling ants, especially carpenter ants (Camponotus spp). In Alberta, as in other areas, they preferred to forage on large (≼25 cm dbh) wood substrates containing carpenter ants. Forest characteristics vary considerably across the range of pileated woodpeckers (Bull and Jackson 1995, Kirk and Naylor 1996) but preference for carpenter ants found in large substrates that are either dead or damaged is a constant. Healthy trees were rarely used. In contrast, I found that habitat selection was very flexible at the territory and stand scales. Most available habitats were used and mature and old forests were not strongly preferred. This suggests that pileated woodpeckers select foraging habitat primarily because it contains food-bearing foraging substrates (Renken and Wiggers 1989, Rolstad et al. 1998), and that most other habitat characteristics are important only insofar as they correlate with foraging substrate (food) density. This conclusion explains why selection at the foraging substrate scale (Conner 1980, Bull et al. 1986, Bull and Holthausen 1993, Flemming et al. 1999) is stronger and more consistent between studies than selection at the forest characteristics scale (Renken and Wiggers 1989, Bull 1987, Mellen et al. 1992, Bull and Holthausen 1993). Pileated woodpeckers specialize on large dead or damaged trees, snags, stubs, logs, and stubs associated with the processes of tree death and decomposition in forests. Predator avoidance behaviour may significantly modify foraging habitat use. Open, young forests regenerated after logging were avoided, probably because of a lack of escape cover (standing trees) in recent cutblocks. Cutblocks with residual trees and regenerated forests >7 m tall were used extensively. The probable importance of trees to facilitate predator avoidance has received little research attention but could be an important issue for forest managers interested in conserving pileated woodpeckers. Patterns for nest and roost tree selection were similar to those described for foraging. The strongest selection was at the cavity tree and position on the cavity tree scales. Pileated woodpeckers preferred large (>35 cm dbh) trees with stem decay, especially living trembling aspen (Populus tremuloides) infected with the Phellinus tremulae fungus. All other forest characteristics associated with cavity trees were less strongly selected. This suggests that pileated woodpeckers select cavity trees primarily because of tree characteristics and that selection at other scales relates to the availability of potential cavity trees. Although there is considerable regional variation in cavity tree characteristics (Conner et al 1975, Bull 1987, McClelland 1979, Peck and James 1983, Mellen 1987, McClelland and McClelland 1999), common features of cavity trees in all regions are that they are large trees with stem decay. Variation in reported tree species, size, and decay characteristics probably reflects relative availability of potential cavity trees between forests more than a difference in preference. Although pileated woodpeckers show regional flexibility in cavity tree selection, they show local preference for cavity tree characteristics when given a choice. In central Alberta and British Columbia boreal and cordilleran forests, living trembling aspens with stem decay were clearly preferred, although balsam poplar (P. balsamifera) and dead conifers were also used. The interrelationships between aspen, P. tremulae, and pileated woodpeckers are significant because aspen is abundant in boreal forests. It grows rapidly, is relatively short-lived, and is prone to infection by P. tremulae, which is endemic throughout the range of aspen. In the absence of other disturbances, stem decay may be the proximate cause of aspen mortality (Hiratsuka et al. 1990). This process acts to benefit pileated woodpeckers in 2 ways: (1) it provides many trees that have suitable characteristics for cavities; and (2) it causes damage and leads to tree mortality, which in turn provides suitable colony-site characteristics for carpenter ants. Similar to foraging habitat selection, predator avoidance was the only habitat factor that may significantly modify cavity tree selection. I believe that the small open area I observed around all cavity trees and the tendency for cavity trees to be located on stand edges probably served to minimize predation risk. The cavity tree provided opportunities for adults to escape avian predators while the open space surrounding it reduced opportunities for predators to exploit the nest. Although the “open spaceâ€? hypothesis has not been previously reported for pileated woodpeckers, it has been noted for the congeneric black woodpecker (Dryocopus martius;


Pwp 2001 04 rpt phdthesis pileatedwoodpeckerhabitatecologyinabfoothills