Cavity nesting birds along an urban-wildland gradient: is human facilitation structuring the bird community?
Urbanization brings dramatic and sudden changes to ecological conditions affecting natural communities. Cavity-nesting birds, both primary and secondary (hereafter PCN and SCN, respectively), may be limited in this novel environment because of reduced abundance of nesting sites (e.g. snags and cavities) and competition for cavities with non-native species. But humans can also directly and indirectly provide nest sites (e.g., nest boxes, crevices on houses), especially for SCN species, potentially partially compensating for negative effects. We investigated whether and how PCNs and humans facilitated the cavity-nesting bird community along a gradient of urbanization. To do so, we 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, we 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.