Vireos

Vireos are 44 species of small arboreal birds that comprise the family Vireonidae, in the order Passeriformes. As it is considered here, the Vireonidae is an assembly of three sub-families: the true vireos or Vireoninae, the shrike vireos or Vireolaniinae, and the pepper shrikes or Cyclarhinae. It should be pointed out, however, that some taxonomic treatments consider these to be separate families.

The vireos only occur in the Americas. Species of vireos range from the temperate forests of Canada, through the rest of North America, Central America, and south to northern Argentina. The northern, temperate species are all migratory, breeding in the northern parts of their biological range, but spending the nonbreeding season in tropical and subtropical forests. Vireos occur in all types of tropical and temperate forests, and in shrubby habitats as well.

Vireos are small birds, ranging in body length from 3.9-7.1 in (10-18 cm). The bill is relatively heavy for a small bird, and the upper mandible has a hook at the tip. Depending on the species, the wings are either long and pointed, or short and rounded, while the legs and feet are short but strong. The plumage of vireos is plain, generally olive-green or gray on the back and wings, and lighter colored on the throat and belly. Species may have eye rings, eye stripes, wing bars, and other diagnostically useful markings.

Vireos glean foliage and branches for their food of insects and spiders, and they may also eat small fruits. Compared with other types of foliage-gleaning birds, vireos are rather sluggish and deliberate in their movements. Vireos generally occur as solitary birds, or in family groups. They are aggressively territorial during the breeding season. The territory is demarcated and defended by loud, melodious songs, consisting of multisyllabic, persistently repeated phrases.

The nest is cup-like, or is an open, pendulous, baglike structure woven of plant fibers, usually located in a horizontal fork of a branch. The clutch size is two to five. The male helps with incubation, and both sexes cooperate in rearing the young birds.