Nest-Site Selection and the Importance of Nest Concealment in the Black-Throated Blue Warbler

Abstract

Nest-site selection of Black-throated Blue Warblers (Dendroica caerulescens) was studied for two summers in a northern hardwood forest in the White Mountains of New Hampshire. Nest placement in the shrub layer was examined to evaluate several alternative hypotheses concerning the selection of nest-site microhabitat. Nest-sites tended to be in dense patches of hobblebush (Viburnum alnifolium) with denser vegetation below 1.5 m and more open vegetation between 1.5-3 m than at random points. Shrub densities at nests and at random points within hobblebush patches were, in nearly all cases, statistically indistinguishable. Nests were placed in large patches of shrubs much more often than at random points and in larger patches of hobblebush more often than at random points within hobblebush patches. Black-throated Blue Warblers placed their nest in more concealed locations compared to empty nests placed at random locations. Concealment indices of successful and depredated nests, however, were not significantly different from one another. Several reasons why this species chose particular microhabitats within the shrub layer are discussed.