Breeding season microhabitat use by Cerulean Warbler (Setophaga cerulea) in an experimentally-managed forest

Cerulean Warbler (Setophaga cerulea) populations have undergone significant declines over the past four decades, and the species is considered state endangered in Indiana. These migratory songbirds nest in the canopies of mature deciduous forests throughout eastern North America, but local microhabitat preferences vary by geography and scale, and may influence differential nest success rates across the breeding range. Between 2010 and 2015, we searched for nests, mapped out male territories, and monitored nest success in the Hardwood Ecosystem Experiment in central Indiana. We collected data on a variety of microhabitat features at the territory, nest patch, and nest site levels, as well as at non-use points, and evaluated the influence of several characteristics on nest success. We evaluated nest patch and territory attributes using binomial generalized linear models and nest success with a generalized linear mixed-effects model. Cerulean Warblers selected nest patches characterized by taller trees and located on ridges and valleys rather than on mid-slopes, while males selected territories that were located on steeper northeast-facing slopes closer to roadways than random points. Successful nests were placed lower in the nest tree and were found in areas with shorter trees; several features selected by Cerulean Warblers at the nest patch and territory levels were associated with lower nest success rates. As elsewhere in the range, white oak was an important nest tree species. In developing forest management strategies, land managers must strive to balance small-scale microhabitat preferences with the need for regenerating oak-hickory forests to sustain Cerulean Warbler breeding populations.