The effects of local- and landscape-level habitat variables on avian cavity-nesting species abundance

Background/Question/Methods

In the last four decades, the southeastern United States has experienced a regional land-cover transformation trend of forest-cover loss, driven by urban development and land-use demands, with the pine-and-mixed-hardwoods of the Piedmont region experiencing the greatest net change. Consequently the Brown-headed Nuthatch (Sitta pusilla) and other avian cavity-nesting species are among the most threatened as their reproductive success is thought to depend on mature old-growth pine stands.  Thus data are needed to aid the development of conservation strategies for suitable remaining habitat, especially in novel urban and managed ecosystems.  We conducted point-count surveys and utilized “unmarked” package in program R to compare binomial-Poisson mixture models to determine the relative effect of patch- and landscape-scale drivers of estimated species abundance.

Results/Conclusions

Model-averaged estimates yielded predictive values for local within-patch variables (canopy height and tree density) and landscape variables (patch size, amount of pine within 100m of the patch) in seven cavity-nesting species including nuthatches, woodpeckers, titmice, and chickadees.  Our data show that predicting abundance in altered and managed forest systems is complex, with both foreseeable and unanticipated drivers of species distribution. Brown-headed Nuthatches (S. pusilla) and Hairy Woodpeckers (Picoides villosus) were more abundant in larger patches.  Red-bellied Woodpeckers (Melanerpes carolinus) were more abundant when there was more pine in the landscape. Both Red-bellied (M. carolinus) and Downy Woodpeckers (Picoides pubescens)were more abundant in patches with greater canopy height.  The higher abundance of Brown-headed Nuthatches (S. pusilla) in urban and peri-urban landscapes suggests that many urban ecosystems are of unexpected high quality and may provide an important refuge for biodiversity.  By identifying factors that limit or predict abundance for ecologically sensitive and for generalist species, we can build a comprehensive model of species occurrence and contribute to regional conservation efforts of both habitat and biodiversity, even in human-dominated systems.