Page 76

Rolstad et al. in review). Preference for relatively sound sapwood, smooth bark, and no branches near the cavity site may also be related to predator avoidance. Open areas would make it easier to see avian predators, especially the northern goshawk (Accipiter gentilis), which may take young at nest entrances (Bull and Holthausen 1993). Sound sapwood would help to prevent nest destruction by larger mammals such as bears. It has been suggested that pileated woodpeckers sometimes use utility poles as nest trees (Dennis 1964, Millar 1992) because they have few opportunities remaining in forests that have been impacted by human activities (Millar 1992, P. Ohanjanian, personal communication). I propose that a more likely alternative explanation is that pileated woodpeckers prefer openings around their nest trees to reduce predation risk, and that they are simply exploiting an opportunity when they nest in utility poles. Short (1979) agreed with this hypothesis and also thought that openings could serve to reduce competition from other species for cavities. I found strong relationships between habitat quality variables and pair territory size. Smaller territories had higher densities of potential foraging substrates and cavity trees, and lower proportions of short (â&#x2030;¤7 m) forest cover. Pileated woodpeckers in my study area had much larger territories than previously reported. Possible explanations for this in comparison to other regions include differences in winter severity and density of foraging substrates, which are probably directly related to food density. However, large territory size did not result in a reduction of fitness. Productivity and survival in my study area were higher than previously reported for pileated woodpeckers, and territory size and other habitat characteristics were independent of productivity and survival. This suggests that pileated woodpeckers are able to maintain fitness across a habitat quality gradient by increasing territory size. As the radiotelemetry portion of my study investigated habitat relationships with fitness across a relatively narrow gradient of forest area and forest age, further work is needed to determine habitat quality thresholds that would affect fitness. However, my cavity tree and productivity results, which were collected over a larger area, suggest that pileated woodpeckers can successfully reproduce and occupy landscapes with low (<10 %) amounts of forest cover. Another observation that may be important to this issue is the co-occurrence of pileated woodpeckers and their main predator, the northern goshawk. Goshawks do not prosper in landscapes with large amounts of young forest or non-forest (Reynolds et al. 1992, Squires and Reynolds 1997). Pileated woodpeckers that exploit such landscapes may be able to do so in the absence of their main predator and adult survival may actually increase when compared to birds living in forested landscapes that are exposed to goshawk predation. Territory size, productivity, and adult survival were quite stable over the study period. Factors that could influence this apparent balance include climatic factors, changes in prey populations (especially carpenter ants), and changes in the abundance of foraging substrates. Inclement spring weather may influence nesting onset (K. Aubry and C. Raley, personal communication) and clutch size (Chapter 3), and winter severity could affect adult survival. Pileated woodpeckers take advantage of temporary food resources such as bark beetle outbreaks (Bull et al. 1986) but little is known about relative population stability of carpenter ants, which are their main prey. Carpenter ant populations may increase if suitable wood substrates are abundant, but other factors may also influence populations (Sanders 1964, 1970). Under natural disturbance regimes, substrates suitable for carpenter ants (and pileated woodpecker foraging) are most abundant for a few decades following major fire disturbance and in mature/old forests that have started to accumulate dead wood through endemic mortality (Lee et al. 1995). If pileated woodpeckers are able to maintain fitness across a habitat gradient, the main effect of reductions in habitat quantity and quality appears to be a direct effect on population density, since territories are vigorously defended against other pileated woodpecker pairs. Declining habitat quality leads to larger territories, which in turn reduces the overall population density. In Alberta foothills forests pileated woodpecker populations appear to be structured in adjacent nonoverlapping territories defended by mated pairs, with non-territorial birds existing as floaters within and between territories. Annual territories, defended all year, were much larger than the area used during the nesting season. I believe that large annual territories were needed to support winter habitat requirements (Chapter 2). Pileated woodpeckers have generally been considered limited by nesting season requirements (Bull and Jackson 1995). I agree with McClelland (1979), who thought that reduced


Pwp 2001 04 rpt phdthesis pileatedwoodpeckerhabitatecologyinabfoothills

Read more
Read more
Similar to
Popular now
Just for you