the blue-headed vireo (vireo solitarius) is a neotropical migrating song bird found in north and central america .
there are currently two recognized sub-species 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 and has therefore been classified as a "least concern" species in 2004.the blueheaded 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 birds 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 126-148 millimetres (4.9-5.8 in) .
the wingspan is usually found to be around 200-240 millimetres (7.9-9.4 in) and their weight is typically 13-19 grams (0.4-0.67 oz) .
there are some subtle differences between the v .
s .
solitarius and v .
s .
alticola sub-species .
v .
s .
alticola , the larger of the two sub-species , has a darker back with a larger wing length of 76-83.5 millimetres (3-3.3 in) and a bill length of 11-12 millimetres (0.43-0.47 in) .
conversely , v .
s .
solitarius is slightly smaller with a wing length of 69.2-77.8 millimetres (2.73 in), and a bill length of 8.9-11 millimetres (0.3-0.4 in) .
the average lifespan of the blue-headed vireo in the wild has been measured to be approximately 7 years and 5 months .
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 .
originally considered its own species , in the 1950s , based on morphological data , the two blue-headed vireo sub-species were grouped with the plumbeous vireo (vireo plumbeus) and the cassins vireo (v .
cassinii) .
this group of vireos was collectively known as the solitary vireo (vireo solitarius) .
this taxonomy prevailed until 1997 when new molecular data on the solitary vireo showed that the three birds were in fact distinct species .
the taxonomy was then changed back and the blue-headed vireo was once again its own species .
there are two recognized subspecies of blue-headed vireo:native to north america , the blue-headed vireo enjoys a large breeding range that expands over an immense area of canada and northern united states .
the breeding range of v .
s .
solitarius expands from north eastern 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 then south , mainly in the appalachian mountains , to northern georgia .
at the end of the breeding season , the blue-headed vireo will migrate south to its overwintering area .
v .
s .
solitarius has the longest migration of the two sub-species and inhabits an overwintering area that includes east and south mexico to northern central america .
v .
s .
alticola migrates across a shorter distance to an area that expands from south-eastern virginia to texas .
during breeding , the vireos prefer temperate forests that are found at higher elevations where it is cooler .
evergreen forests with spruce , fir , hemlock and pine with deciduous growth such as alder shrubs , willow shrubs , poplar , birch or maple trees are the habitat of choice for the small bird .
during the winter , vireos inhabit woods of pines , hardwoods and mixed trees .
they are also found in coastal and flood plain swamps as well as low shrub thickets .
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 .
during migration , blue-headed vireos are often found to flock with groups of different sparrow species but never with members of its own species .
when arriving to the breeding grounds in mid-april , many deciduous trees have not begun to leaf out , prompting the migrating blue-headed vireos to nest in the evergreen hemlock tree .
recently , however , some of the native habitat of the blue-headed vireo has been deteriorated by an invasive asian insect , the hemlock woolly adelgid (adelges tsugae) .
these true bugs feed on the evergreen hemlock trees which leave the migrating vireos with fewer habitats to form breeding nests until the deciduous trees begin to leaf .
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 both defence of nests , interspecies competition , and communication between breeding pairs while foraging .
when foraging during the breeding season , mating pairs will constantly communicate with each other using a series of high pitched chirping calls .
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 .
usually , foraging will occur 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 .
typical prey consist mostly of caterpillars , butterflies and moths but may also include tree bugs , stink bugs , beetles , bees , ants , flies , dragonflies , grasshoppers , crickets , spiders and on some occasions , snails .
fruit feeding is seen mostly during migration and overwintering , especially when overwintering in more tropical regions where fruit is more abundant or if the insect populations are low .
this large and varying diet can be attributed to a more opportunistic feeding strategy , allowing the blue-headed vireo to be plastic in its ability to adapt to resource availability .
when arriving to the breeding ground in mid-april , male vireos will hold large , individual territories that can range from 1.5-6.6 hectares .
females will choose a male to mate with depending on the males 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 .
once a female has approached a suitable mate , the male will perform a series of mating rituals .
typically , these rituals involve the male building a courtship nest for the female .
the courtship nest is not a full nest but is most likely an attempt by the male to display his ability to gather resources .
once the pair is established , the courtship nest is abandoned .
mating does not occur for approximately 2 weeks after pair has been established .
during this time , the male and the female will cooperatively build a breeding nest which is typically formed between forking mid level branches of evergreen and deciduous trees and are composed of twigs and foliage .
breeding nests are not reused in following breeding seasons .
in one breeding season , a female will lay 3-5 eggs at a rate of 1 egg/day .
eggs are creamy white with brown or black spots and are 17-23.1 millimetres x 13.3-15.8 millimetres (0.055-0.075 in x 0.04-0.05 in) .
eggs are incubated for approximately 14 days after being laid .
once hatched , infants are highly altricial , often having little to no downy feathers .
infants open their eyes during day 5 or 6 and will begin to fledge after 14 days for a total chick-rearing period of 28 days .
once the young are able to leave the nest , the parents will split the juveniles and separate .
juveniles are usually dependant on the parent to provide food for approximately 1 month after leaving the nest .
if the nest is destroyed or a mating pair produces a failed brood , an attempt to re-nest may occur but this ultimately depends on when the brood fails .
if the brood fails early on in the season , there is a higher chance that the pair will re-nest , if later , the pair will often separate and not attempt a second brood .
blue-headed vireos have an atypical breeding strategy when compared to other closely related species .
blue-headed vireo males will assist their mates with daytime incubation of eggs , nest construction , and feeding of the chicks (morton et al .
1998) .
also , unlike most bird species , female blue-headed vireos do not attempt any extra-pair copulations with other males .
likewise , males make no attempt to guard females from other males , making blue-headed vireos both socially and genetically monogamous birds .
the increase in male blue-headed vireo parental care is thought to be attributed to the levels of testosterone and prolactin found in the plasma of these birds during the mating season .
testosterone , a steroid hormone that is secreted by the testes , has been found to increase aggressive behaviour in male birds during the breeding season 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 vireos 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 .
blue-headed vireos have a low nesting success rate , with about 10-30 percent of hatchlings surviving .
common predators of the blue-headed vireo eggs and chicks include blue jays , crows , and squirrels .
often , when a nest is attacked by a blue-jay , the male and female will execute coordinated attacks on the approaching predator .
this usually involves one partner swooping towards the blue-jay while calling the mate who will then execute a second attack .
blue-headed vireos have also been known to nest close to nesting raptorial birds that do not prey on them .
it is thought that they use the raptors as protection from squirrels that are , in turn , hunted by the these bird of prey .
the largest contributor to the low nesting success rate of the blue-headed vireo is most likely the parasitic brown-headed cowbird (molothrus ater) .
these birds will often lay their large eggs overtop the existing brood , causing the parents to feed the cowbird which will ultimately starve the infant vireos .
