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Yellow Warblers breed in a range of habitats in eastern North America, including thickets, swamp edges, streamsides, second growth woods, orchards, and gardens. In the West, they are largely restricted to streamside thickets. On their tropical wintering grounds, Yellow Warblers are found in semi-open country, woodland edges, and towns.
Yellow Warblers feed mainly on insects; up to two thirds of the diet may consist of caterpillars. They forage alone on their wintering grounds and defend a winter feeding territory.
Males defend nesting territories by singing and sometimes perform fluttering flight displays. The male courts the female by actively pursuing her for 1 to 4 days. The nest, which is built largely by the female, is placed in an upright fork of a shrub, small tree, or thicket from 1 to 18 m above the ground. Females may steal nest material from other nests. Females lay 4 to 5 (sometimes 3 or 6) eggs that are greenish white with a variety of specks or spots of brown, olive, and gray. Eggs are incubated by the female for 11 to 12 days. The male feeds the female on the nest. Young are fed by both parents, but especially the female. They leave the nest 9 to 12 days after hatching.
Yellow Warbler nests are often parasitized by Brown-headed Cowbirds, which lay their eggs in the warbler's nest. In some areas. Yellow Warblers may recognize the problem, build a new floor over the eggs, and lay a new clutch of their own (or, alternatively, simply abandon the nest). In one reported case, cowbirds laid eggs on 5 visits, but the warblers built a new floor after each visit.
Migration is mostly by night. Fall migration begins quite early, with many Yellow Warblers moving south during August.
(Kaufman 1996; AOU 1998; Dunn and Alderfer 2011)