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They are typically dull-plumaged and greenish in color, the smaller species resembling wood warblers apart from their heavier bills. They range in size from the Chocó vireo, dwarf vireo, and lesser greenlet, all at around 10 cm (4 in) and 8 g (0.28 oz), to the peppershrikes and shrike-vireos at up to 17 cm (6.7 in) and 40 g (1.4 oz).
Distribution and habitat
Most species are found in Middle America and northern South America. Thirteen species of true vireos occur farther north, in the United States, Bermuda and Canada; of these, all but Hutton's vireo are migratory. Members of the family seldom fly long distances except in migration. They inhabit forest environments, with different species preferring forest canopies, undergrowth, or mangrove swamps.
A few species in the genus Vireo have appeared on the eastern side of the Atlantic as vagrants to the Western Palearctic.
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Behaviour
The resident species occur in pairs or family groups that maintain territories all year (except Hutton's vireo, which joins mixed feeding flocks). Most of the migrants defend winter territories against conspecifics. The exceptions are the complex comprising the red-eyed vireo, yellow-green vireo, black-whiskered vireo, and Yucatan vireo, which winter in small, wandering flocks.
Voice
Males of most species are persistent singers. Songs are usually rather simple, monotonous in some species of the Caribbean littoral and islands, and most elaborate and pleasant to human ears in the Chocó vireo and the peppershrikes.
Breeding
The nests of many tropical species are unknown. Of those that are known, all build a cup-shaped nest that hangs from branches. The female does most of the incubation, spelled by the male except in the red-eyed vireo complex.
Feeding
All members of the family eat some fruit, but mostly insects and other arthropods. They take prey from leaves and branches; true vireos also flycatch, and the gray vireo takes 5% of its prey from the ground.
Systematics
The family Vireonidae is related to the crow-like birds in family Corvidae and the shrikes in family Laniidae as part of superfamily Corvoidea. As currently circumscribed, the family is made up of eight genera.
Traditionally, the family was considered to include four New World genera containing the true vireos (Vireo), the greenlets (Hylophilus), the shrike-vireos (Vireolanius), and the peppershrikes (Cyclarhis). However, phylogenetic studies found Hylophilus to be polyphyletic, with the greenlets split into three distinct groups - the "scrub" greenlets in a restricted Hylophilus, the "canopy" greenlets in resurrected genus Pachysylvia, and the tawny-crowned greenlet in new genus Tunchiornis.
In addition, biochemical studies have identified two babbler genera (Pteruthius and Erpornis), which may be Old World members of this family. Observers have commented on the vireo-like behaviour of the Pteruthius shrike-babblers, but apparently no one suspected the biogeographically unlikely possibility of vireo relatives in Asia. Some recent taxonomic treatements, such as the IOC taxonomy followed here, include Pteruthius and Erpornis in the Vireionidae, whereas others place them in their own families Pteruthidae and Erpornidae.