Vireos

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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 and greenlets (Vireoninae), the shrike-vireos (Vireolaniinae), and the peppershrikes (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 4-7.1 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 cuplike, or is an open, pendulous, bag-like 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.

North American species of vireos

A total of 12 species of vireos breed regularly in the United States or Canada. All of these are in the genus Vireo, and all are migratory, spending their nonbreeding season in Mexico or further south in Central America, or in the case of the red-eyed vireos, in Amazonia.

The red-eyed vireo (Vireo olivaceus) is a widespread species, occurring in deciduous forests over much of the continent, except for parts of the southwestern United States. The red-eyed vireo is an abundant species, and is one of the most commonly netted birds at mist-netting sites in eastern North America, where bird migration is studied.

The warbling vireo (Vireo gilvus) breeds in much of temperate North America and south into Mexico. The warbling vireo is a secretive bird, and males can be so confident in their camouflage that they will sing from the nest while incubating their clutch.

The solitary or blue-headed vireo (Vireo solitarius) breeds in mixed hardwood-conifer forests through much of north temperate North America. This is one of the only species of vireo whose wintering range commonly includes the southern United States.

The white-eyed vireo (Vireo griseus) is an abundant species of moist deciduous forests and forest edges in the eastern United States. The yellow-throated vireo (Vireo flavifrons) has a bright yellow throat and breast, and is perhaps the most attractive of the North American species. This relatively uncommon species breeds in deciduous forests throughout the eastern United States.

Vireos elsewhere

Most species of true vireos occur in subtropical, tropical, and montane forests of Central and South America, but these species are too numerous and diverse to deal with here in any detail. One representative is the ashy-headed greenlet (Hylophilus pectoralis), a species found in the tropical forests of Guyana, Surinam, Brazil, and Bolivia.

The shrike-vireos are relatively stout, tropical birds with a heavy, hooked bill. The chestnut-sided

KEY TERMS

Nest parasite —A species that lays its eggs in the nests of other species. The host raises the parasitic egg, and usually does not raise any of its own babies.

shrike vireo (Vireolanius melitophrys) occurs in tropical rainforests of Mexico and Guatemala.

The peppershrikes also are stouter than the true vireos, and have a laterally compressed, hooked bill. The rufous-browed peppershrike (Cyclarhis gujanensis) occurs in open forests from Mexico to Argentina.

Vireos and people

In spite of their small size, vireos are economically important. Sightings of these birds are avidly sought by birdwatchers. Birding is a nonconsumptive field sport, and is increasing rapidly in popularity. Birding and related activities such as bird feeding have large economic impacts, and give great aesthetic pleasure to many people.

Unfortunately, the populations of many species of birds that are the targets of these activities, including vireos, are declining greatly because of human activities. This is especially true of many species native to North America.

Vireos and other birds that share their habitat are at risk from changes occurring in both their breeding and wintering ranges. There have been tremendous decreases in the areas of mature forests that most vireos require for breeding in North America. The vireos are affected directly by these losses of area of their essential habitat, as well as by indirect effects associated with the fragmentation of much of the remaining habitat into small woodlots.

Small, isolated, habitat “islands” are highly influenced by their proximity to edges with younger habitat. This circumstance exposes vireos and other birds of the forest-interior to a greater intensity of predation, and to the disastrous effects of nest-parasitism by the brown-headed cowbird (Molothrus ater). Many ornithologists believe that these factors are causing large declines in the populations of numerous species of migratory forest birds, including many of the vireos.