The Foraging of Small Populations of Yellow Warblers and American Redstarts

Abstract

I studied the foraging of Yellow Warbler (Dendroica petechia) and American Redstarts (Setophaga ruticilla) on several small islands of varying vegetational composition off the coast of Maine. Both species used deciduous growth more than predicted by chance alone. Yellow Warblers foraged slightly lower than Redstarts; Redstarts used dead limbs and hawked for insects more often than Yellow Warblers. On islands with mixed forests both species fledged young regularly; there they set up largely exclusive territories. Partitioning appeared to be in response to certain vegetational characteristics; little evidence was obtained that direct interactions were of major importance in this regard. Most male Redstarts in the latter areas were adults; most males on small spruce—clad islands had first—year plumage. Only one species occupied any small spruce—clad island, and seldom did it fledge young. Usually a male did not attract a female in such cases. No clear correlation occurred between the size of the standing insect crop, the tendency of these species to occupy an island, and their ability to fledge young. While Parula (Parula americana), Myrtle (Dendroica coronata), and Black—throated Green (D. virens) warblers displayed a highly developed vertical division of space and a well—defined interspecific social hierarchy (Morse 1971), Yellow Warblers and Redstarts divided space horizontally and did not demonstrate a well—defined interspecific social hierarchy. Correspondingly, one may find up to three species of spruce—woods warblers on an island too small to support more than one pair of a given species, but never both Yellow Warblers and Redstarts in such a situation.