The blue-headed vireo (Vireo solitarius) is a migrating song bird found in North and Central America. There are currently two recognized subspecies that belong to the blue-headed vireo. It has a range that extends across Canada and the eastern coast of the United States, Mexico, and some of Central America. It prefers large temperate forests with a mix of evergreen trees and deciduous under growth.
As the name suggests, the blue-headed vireo is characterized by its blue-grey head and bold yellow wing bars. Both sexes are very similar in plumage and size. Juveniles also have a similar plumage.
Populations of the blue-headed vireo have been steadily increasing since the 1970s. Therefore, in 2004, the species was classified as a "Least Concern" species.
Description
The blue-headed vireo has similar plumage year round and does not drastically change its appearance during the breeding season. It can be characterized by its olive green upper body, two bold yellow wing bars down the edge of its wing, and a deep blue-grey crown from which it gets its name. The juvenile plumage of immature blue-headed vireos is not distinct but very similar to the adult plumage. While this bird's appearance is similar to the closely related Cassin's vireo (Vireo cassinii), the Cassin's vireo can be differentiated by its lighter yellow patches, a smaller, thinner bill, and a brownish-grey crown. There is little to no sexual dimorphism between males and females as both are similar in plumage colouration and size.
Like most Vireos, the blue-headed vireo is a relatively small bird with a length of . The wingspan is usually found to be around and their weight is typically . There are some subtle differences between the V.s. solitarius and V.s. alticola subspecies. V.s. alticola, the larger of the two subspecies, has a darker back with a larger wing length of and a bill length of . Conversely, V.s. solitarius is slightly smaller with a wing length of , and a bill length of .
There are two recognized subspecies of blue-headed vireo: During the winter, blue-headed vireos inhabit mixed woods of pines and hardwoods. They are also found in coastal and flood plain swamps and low shrubby thickets. The density of the population ultimately depends on the type of forest being inhabited.
Behaviour
Vocalizations
thumb|Song of a male blue-headed vireo in [[Minnesota]]
The primary song of the blue-headed vireo is a simple, slow, monotonous tune that is often confused with the faster red-eyed vireo (Vireo olivaceus) song. The primary song is sung only by the males and usually only during the breeding season. The males become quite vocal during the breeding season, with the song frequency reaching its height prior to mating and then decreasing after copulation. Both sexes share a series of calls that are used in defence of nests, interspecies competition, and communication between breeding pairs while foraging.
Diet
Blue-headed vireos are mainly insectivorous birds but are also known to eat fleshy berries and fruit. They are equipped with short, strong bills used for processing insect prey with a tough carapace such as beetles. Foraging usually occurs in the mid level of trees. A foraging blue-headed vireo will hop from branch to branch on the same or to an adjacent tree and will then fly towards its prey to capture it.
Reproduction
When arriving to the breeding ground in mid-April, male vireos will hold large, individual territories that can range from . Females will choose a male to mate with depending on the male's ability to defend and control a large territory. While defending their territory from other males, the male vireos will sing a primary song to attract females. Undecided females can usually be seen flying along the edges of competing male territories; usually this will force the two males into direct conflict for the right to copulate with the female. and that these males display less parental care if their testosterone levels are maintained. Conversely, prolactin is a peptide hormone that is regulated by the hypothalamus that has been found at higher levels in birds that do the majority of incubation. It was found that in the early breeding season, male blue-headed vireos have a low testosterone level and will maintain this low level at all stages of the season. This low level does not affect the Vireo's ability to compete with other males for territory however, as a high testosterone level has been found to not be necessary for territory establishment. Prolactin levels of the males were found to be high very early on in the breeding season and are maintained for the duration of the season. This high level of prolactin along with a low testosterone level, explains why male blue-headed vireos are so involved in the construction of the nest, daytime incubation, and feeding of the nestlings.