Blue-headed Vireo

Identificazione

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.<p>Like most Vireos, the blue-headed vireo is a relatively small bird with a length of 126–148 mm (5.0–5.8 in). The wingspan is usually found to be around 200–240 mm (7.9–9.4 in) and their weight is typically 13–19 g (0.46–0.67 oz). 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 76–83.5 mm (2.99–3.29 in) and a bill length of 11–12 mm (0.43–0.47 in). Conversely, V.s. solitarius is slightly smaller with a wing length of 69.2–77.8 mm (2.72–3.06 in), and a bill length of 8.9–11 mm (0.35–0.43 in).<p>The average lifespan of the blue-headed vireo in the wild has been measured to be approximately 7 years and 5 months.

Tassonomia

The blue-headed vireo is placed in the genus Vireo, a group of small migrating passerine birds that are characterized by their usually dull greenish colouring, and a simple, monotonous song. Members of this group are usually characterized by the presence or absence of eye-rings, eye-strips, and wing bars. There is no sexual dimorphism between sexes in this group.<p>Though it had been previously classified as a separate species, the blue-headed vireo along with the plumbeous vireo (Vireo plumbeus) and the Cassin's Vireo (V. cassinii) were lumped in the 1950s into a single species, based on morphological data. This group of vireos was collectively known as the "Solitary Vireo" (Vireo solitarius). This taxonomy prevailed until 1997 when new molecular data showed that there were in fact three distinct species. The taxonomy reverted to that prior to the 1950s, and the blue-headed vireo was once again its own species.<p>There are two recognized subspecies of blue-headed vireo:

Nome scientifico

Vireo solitarius

Autorità

(Wilson, A, 1810)

Ordine

Passeriformi

Famiglia

Vireonidi

Genere

Vireo

Areale e habitat

Native to North America, the blue-headed vireo enjoys a large breeding range that extends over an immense area of Canada and northern United States. The breeding range of V.s. solitarius extends from northeastern British Columbia across Alberta, Saskatchewan, Manitoba, Ontario and Quebec and down to southern Pennsylvania and New Jersey. V.s. alticola has a distinct breeding range that includes western Maryland and south, mainly in the Appalachian Mountains, to northern Georgia. At the end of the breeding season, the blue-headed vireo migrates south to its overwintering area. V.s. solitarius has the longest migration of the two subspecies and inhabits an overwintering area that includes eastern and southern Mexico to northern Central America. V.s. alticola migrates across a shorter distance to an area that extends from southeastern Virginia to Texas.<p>Blue-headed vireos prefer to breed in cool temperate forests, which in the southern part of its breeding range are found at higher elevations. Evergreen forests with spruce, fir, hemlock, and pine mixed with deciduous growth such as alder shrubs, willow shrubs, poplar, birch or maple trees are the habitat of choice. 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.<p>Year round, even during the breeding season, population density is somewhat low and spread out. Overwintering population densities are usually found to be lower than in the breeding season. The density of the population ultimately depends on the type of forest being inhabited.<p>During migration, blue-headed vireos are often found to flock with groups of different sparrow species but rarely with members of its own species.<p>When blue-headed vireos arrive on their breeding grounds, typically in mid-April, many deciduous trees have yet to leaf out, leading them to nest in evergreen trees including hemlocks (Tsuga). Recently, however, many hemlocks in eastern USA have been killed by an invasive Asian insect, the Hemlock woolly adelgid (Adelges tsugae), leaving the vireos with fewer nesting options until spring leaf opening begins for deciduous trees.

Areale riproduttivo

America settentrionale orientale

Areale non riproduttivo

Costa Rica; Cuba

Migrazione

Migratori completi

Comportamento

Comportamento migratorio: Migratori completi.

Conservazione

Tendenza della popolazione: in aumento.

Tendenza della popolazione

in aumento