An Experimental Study of Nest-Site Selection by Yellow Warblers

We studied nest-site selection of Yellow Warblers (Dendroica petechia) in a riparian community on the Arapaho National Wildlife Refuge, Colorado, from 1981-1984. Our methodology included (1) comparing nest sites selected by warblers to the structure and vigor of the woody community available as potential sites, (2) experimentally removing shrubs used for nest sites and describing new sites selected, and (3) evaluating the reliability of habitat descriptors measured at nest sites across three years of pooled nest-site information. Warblers built nests proportionally among the seven species of willow (Salix) shrubs in the community. Of the original 34 descriptors of vegetation structure and vigor, 13 were considered insignificant to Yellow Warblers in used-vs.-available comparisons, an additional seven were eliminated comparing nest sites before and after the experimental manipulation of the shrub community, and another three were eliminated with a comparison of nest sites across all three years. In all, 11 descriptors remained after the multiple analyses. Virtually all descriptors of structure and vigor of bushes where warblers built nests were eliminated with the multiple analyses. Birds selected nest sites based primarily upon descriptors of the vegetation patch, the most powerful being those that defined horizontal patterning of bushes within the patch. We conclude that (except for the immediate branch structure supporting the nest) Yellow Warblers select nest sites based upon patch characteristics surrounding the bush where the nest was built rather than on characteristics of the nest bush itself. This behavior probably reflects selective pressures to reduce rates of nest predation and brood parasitism by concealing nests in larger stands of shrubs. Finally, the results of the multiple tests led us to conclude that published information on passerine habitats and habitat selection likely contains a large component of statistically valid, but biologically meaningless, relationships.