Cavity-Nesting Bird Interactions In The Urban-Suburban Gradient
Tomasevic, Jorge A.
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Urbanization brings dramatic and sudden changes to ecological conditions affecting natural communities. I studied potential effects on cavity-nesting birds, both primary and secondary (hereafter PCN and SCN, respectively), which may be limited in these novel environments because of reduced abundance of nesting sites (e.g. snags and cavities) and competition for cavities with non-native species. I addressed community-wide effects of urbanization on cavity nesting birds, and then focused on the ecology of a particular cavity-nesting species, the Pileated Woodpecker (Dryocopus pileatus). Humans can potentially compensate for negative effects of urbanization by directly and indirectly providing nest sites (e.g., nest boxes, crevices on houses), especially for SCN species. I investigated whether and how PCNs and humans facilitated the cavity-nesting bird community along a gradient of urbanization. To do so, I estimated the abundance of cavity-nesting species between 1998 and 2010 at 135, 1-km2 sites that differed in the degree of urbanization (0–100% forest cover). Also, I found 367 nests on a subset of 31 sites. PCNs (n=67 nests) nested mostly on snags (98.5%), while native SCNs (n=141) used both natural (71.63%) and anthropogenic (28.37%) cavities. Non-native SCNs (n=159 nests) used mostly anthropogenic cavities (98.11%). PCN abundance facilitated native SCN abundance on sites with more than 12% forest cover at 1-km2 scale, but not at less forested sites. There, native SCNs nested primarily (59%) in anthropogenic cavities. Human facilitation allowed native SCNs to successfully use and reproduce where snags were scarce, changing the composition and structure of the cavity-nesting bird community within the most urbanized sites. Flexible nest site selection and human facilitation provide new opportunities for native cavity-nesting birds in a rapidly changing world. I studied how a purported sensitive species, the Pileated Woodpecker (Dryocopus pileatus), adapts to rapid and extensive land-cover change as urbanization reduces and isolates forest. From 2009 – 2013, I used radio-telemetry to determine the annual home range size and habitat use of 17 individuals in 9 suburbs that varied in their level of urbanization (ranging 5 – 90% forest remaining). I used Concentration of Use and Resource Utilization Functions to examine vegetative characteristics used by woodpeckers at the landscape (i.e. 1 km2) and local (i.e. 1/3 ha) levels. The average suburban woodpecker home range was significantly smaller than expected based on latitude. Pileated woodpeckers significantly concentrated their use of the landscape on native forest (coniferous and deciduous), as well as light and medium urbanized areas. Highly urbanized areas were seldom used. Resource use was highest along edges between forest and light and medium urbanized areas, and in forests with increasing mean diameter of dominant hardwood species. My results not only indicate the adaptability of a species that has traditionally been considered a mature forest specialist, but they also suggest that maintaining forest cover above 20%, retaining large deciduous trees and snags in public green spaces and yards, and providing feeders would improve the biodiversity of suburban areas. Finally, home range, territory and core areas are concepts that have been used to describe space use. However, little research has been done to understand potential spatial relationships between them. And while the relative importance of different areas of the home range has been addressed with utilization distributions; there is a lack of such analysis for territories. I propose a behavior-based approach to determine areas of importance within the territories, that I defined as highly-defended areas. I studied the spatial ecology of a territorial species, the pileated woodpecker, and the relationships between their home range, territory, core areas, and highly-defended areas. I found significant spatial overlap between male and female woodpeckers of the same breeding pair, but little overlap between same sex individuals on neighboring home ranges. On average, territories represented 69.63% ± 0.06% of the home ranges, and highly-defended areas were 34.3% ± 0.03% of the home range. My definition of a highly-defended area was useful in determining the portions of the territory that were important for the birds. Though more objective that other proposed methods, my approach is contingent upon the types of behaviors surveyed. In my case, highly-defended areas contained a significant proportion of the roost sites for pileated woodpeckers, a resource that may affect survivorship, especially in winter. This approach could be useful to further incorporate behavior on the study of the spatial ecology of species.
- Forestry