Songbird community varies with deer use in a fragmented landscape

Abstract

Consumption of understory foliage by abundant ungulates can reshape forest structure and thus induce corresponding ecosystem changes. In forest songbirds, a negative response to deer browsing was documented for species that use understory foliage, although this association remains poorly understood at the community level. Such knowledge is especially important in eastern North America where deer populations have recently increased substantially. Our primary objective was to examine correlations between deer and songbirds in coastal Virginia (n=92 sites, 2010–2013), a region with heavy forest fragmentation and abundant deer. The secondary objectives were to compare coastal surveys to an inland region (n=99 sites, 2012) with fewer deer and lower fragmentation, and to explore links between fragmentation and deer. We predicted that densities of songbirds that use understory foliage (“deer-sensitive”) would correlate negatively with deer fecal pellets – our measure of relative deer use. The estimate of median pellets inland (0ha−1, range 0–19,600pelletsha−1) was significantly lower than on the coast (2,014ha−1, range 0–28,193ha−1). We found a negative correlation with deer-sensitive birds on the coast (rs=−0.35, P=0.002), whereas inland, no correlation was evident. Coastal sites had approximately twice as much fragmented forest, which favors deer. Of 26 songbirds in our species-level analysis, we found negative correlations for three species, including one of high conservation concern. These results are consistent with the hypothesis that habitat modification by over-abundant deer promotes measurable changes in bird communities, with conservation implications for declining songbirds.