Yellow Warblers are small, evenly proportioned songbirds with medium-length tails and rounded heads. For a warbler, the straight, thin bill is relatively large. Look for Yellow Warblers near the tops of tall shrubs and small trees. They forage restlessly, with quick hops along small branches and twigs to glean caterpillars and other insects. Males sing their sweet, whistled songs from high perches. Yellow Warblers breed in shrubby thickets and woods, particularly along watercourses and in wetlands. Common trees include willows, alders, and cottonwoods across North America and up to about 9,000 feet in the West. In winter they mainly occur in mangrove forests of Central and South America.
https://www.allaboutbirds.org/guide/Yellow_Warbler/id