Blue-green Warbler

The sky-blue upperparts of the male Cerulean Warbler are difficult to observe in summer: At that season, the birds stay high in the tops of leafy trees in the eastern United States and extreme southern Canada. The bird itself has become harder to observe in recent decades, as its numbers have decreased in parts of its range. Cowbirds, which lay their eggs in the warblers' nests, may be finding their unwitting 'hosts' more easily as forest patches become smaller.