Is nest predation on two endangered bird species higher in habitats preferred by snakes?

Abstract

Higher predation on birds' nests is often associated with habitat fragmentation and edges, but little research has addressed whether these predation patterns result from non-random habitat use by predators. Using 2 endangered bird species, black-capped vireo (Vireo atricapilla) and golden-cheeked warbler (Dendroica chrysoparia), and their primary nest predator, the Texas ratsnake (Elaphe obsoleta), we test the hypothesis that birds nesting in habitats preferred by ratsnakes suffer higher nest predation than those nesting in non-preferred habitats. Ratsnakes did not use their habitat randomly, instead preferring locations with more structure and closer to cover objects and edges. Despite large sample sizes (186 nests and 31 snakes with radio transmitters), however, we did not find clear relationships between snake habitat preference and avian nest survival. Our results, in conjunction with those of another study, suggest that warbler nests were at greater risk if edge was abundant near the nest. Thus, reducing edge could promote nest survival. Additionally, because ratsnakes preferred warbler habitat over vireo habitat, vireos might suffer less predation in larger habitat patches that would increase average distances of nests from the snakes' preferred habitat. In making one bird species' habitat less attractive to ratsnakes, however, we may increase snake predation on other species.