SITE-SPECIFIC AND LANDSCAPE FEATURES ASSOCIATED WITH SHRUBLAND BIRD OCCURENCE IN ANTHROPOGENIC SHRUBLANDS IN THE NORTHEASTERN UNITED STATES

Habitats dominated by low-growing trees and shrubs are becoming increasingly uncommon in the northeastern U.S. Human development, altered natural-disturbance regimes, and forest succession have reduced the quantity and quality of these shrublands. As a result, over half of the shrubland-dependent songbirds in the region have experienced long-term population declines. Anthropogenic shrublands, including regenerating clearcuts, sand and gravel mines, old fields, and transmission line rights-of-way may provide nesting habitat for most shrubland birds; but differences in size, site-specific features, and landscape composition may affect bird use. To assess the features that may influence shrubland bird occurrence in anthropogenic shrublands, I conducted presence/absence surveys of 8 species [alder flycatcher (Empidonax alnorum), brown thrasher (Toxostoma rufum), blue-winged warbler (Vermivora cyanoptera), chestnut-sided warbler (Setophaga pensylvanica), eastern towhee (Pipilo erythrophthalmus), field sparrow (Spizella pusilla), indigo bunting (Passerina cyanea), and prairie warbler (Setophaga discolor)] in 101 sites in southeastern New Hampshire during the 2015 and 2016 nesting seasons. For each shrubland, I measured area, site-specific features (e.g., vegetation height, density, and coverage), and characteristics of surrounding landscape features within different buffer zones. Overall, 67% of the variables in the best models predicting bird occurrence were landscape features and 33% were site-specific features. Bird occurrence at a site was positively associated with the proportion of shrublands in the surrounding landscape, particularly within a 500 m buffer. Occurrence of all species except blue-winged warblers and indigo buntings was negatively associated with the proportion of urban development in the surrounding landscape. Shrubland bird species richness increased with vegetation density until vegetation density became too dense for brown thrashers, field sparrows, and prairie warblers. Occurrence of all species except blue-winged warblers increased with shrubland size. These results provide opportunities to enhance existing anthropogenic habitats to benefit populations of declining shrubland birds.