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Cassin's Vireo (Vireo cassinii)
Distribution

Breeding: Breeds from British Columbia and southwestern Alberta through central Idaho,Cassin's Vireo breeding distribution map across to coastal Washington and Oregon, south to southern California.

Winter: A small number overwinter in southeastern Arizona, winter ranged extends south to Costa Rica.

Breeding habitat

Coniferous and mixed forests, also found in second growth and brush areas on hillsides or along stream bottoms.

 

Conservation status

This species is of moderate conservation importance, primarily because of its small overall range, and association with mature forests of the Pacific Northwest. Its populations appear to be steadily increasing in most parts of its range, however. Understanding precise habitat requirements, especially sensitivity to fragmentation and various silvicultural practices will be important for maintaining healthy populations of this vireo.

Description

Male: Shows all of the traits of the "Solitary Vireo complex," and plumage is roughly intermediate between Blue-headed and Plumbeous Vireos. Solid blue-gray hood contrasts with white spectacles and whitish throat, back olive showing white bars on dark wings tinged with greenish-yellow. Bright yellow sides and flanks, sometimes mixed with green.

Female: Same as adult male.

Juvenile: On average, browner and drabber than adults.

Vocalizations

Song: Consists of rough, burry phrases, very similar to Plumbeous and Blue-headed Vireos. Slightly higher pitched and with more clear phrases than Plumbeous; much overlap, but distinctly lower-pitched and more burry than Blue-headed. Generally depicted as, see you, cheerio, be seein you, so-long, seeya . . .

Calls: A harsh series of falling or steady rapid notes, ship shep shep shep shep shep shep shep.

Foraging strategy

Searches among leaves and stems of bushes and trees, gleaning insects from foliage. Occasionally sallies or jumps for passing insects.

Diet

Nearly entirely insects, including squash-bugs, leaf-bugs, stink-bugs, shield-bugs, leaf-hoppers, tree-hoppers, jumping plant-lice, scales, caterpillars, and moths.

Behavior and displays

Male performs nest-building display while slightly crouched with body horizontal usually without nest material in bill.

Fearless around nest; female described as a "close sitter."

Courtship

Male fluffs conspicuous yellow flank feathers while bobbing, bowing and singing to female.

• Male occasionally starts building one or more nests before pairing.

Nesting

Nest site: Male selects a site, normally low in bushes or trees. Nest placed in a forked twig of a small forest tree. It breeds commonly and builds its dainty nest low in bushes or trees.

Height: Typically 6-15 feet (2-5 meters) high, some nests 32 feet (10 meters) above the ground.

Nest: A basket-like deep cup is constructed of grass, forbs, shredded bark, plant fibers, spider web, and cocoons, often decorated with lichen and lined with fine grass and hair. The nest is placed in a forked branch or twig, suspended by the rim.

Eggs: 3-5 white eggs are lightly spotted with brown.

Incubation period: Little information, 15 days during which both male and female incubate.

Nestling period: Young are altricial and downy at hatching, and are tended by both parents.

Fledgling period: Nestlings are able to fly at 13 days, usually leave the nest at day 14. Parents divide fledged brood and leave nest area.

Broods: Little information, probably double-brooded.

Cowbird Parasitism: Common host to Brown-headed Cowbird. If the cowbird egg is laid first, the parents often build a second floor of the nest to cover it.

Notes: The bird formerly known as the Solitary Vireo was recently split into three separate species by American ornithologists: Cassin's, Plumbeous, and Blue-headed Vireos. Cassin's Vireo occupies the western-most part of the former Solitary Vireo's range.

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