vireo, small, migratory songbird of the New World. Some species nest in the United States, but the majority are tropical. Vireos (also called greenlets) range from 4 to 6 1/2 in. (10.2–16.5 cm) in length; they are greenish above and white or yellowish below and have either stripes above or rings around the eyes. They search methodically through vegetation for insects. Vireos are known for their loud, persistent call and for their cup-shaped nest that is suspended in the fork of a tree limb. American vireos include the red-eyed, white-eyed, warbling, Philadelphia, yellow-throated, and blue-headed, or solitary, vireos. Vireos are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Passeriformes, family Vireonidae, genus Vireo.