Cassin’s Vireo

A subtly handsome songbird with a bold white eyering, Cassin’s Vireo sings a loud, burry song while foraging slowly through the lower and middle parts of trees. It breeds in dry, open forests of far western North America. Outside the breeding season, this species often joins mixed-species flocks of woodland birds. Until 1997, Cassin’s Vireo was lumped with Blue-headed and Plumbeous Vireos into a single species called Solitary Vireo. Cassin’s is less colorful than Blue-headed but richer in olive tones than the mostly gray Plumbeous.

Adult Male (spring/summer)

Upperparts are dull olive-green becoming gray on the crown and auriculars. Face is brownish gray and the bold white ‘spectacles’ are formed by a supraloral stripe and eye-ring. Wings and tail brownish black with 2 broad yellowish-white wing bars. Remiges and rectrices finely edged olive-yellow or grayish olive: outer rectrix finely edged in white. Throat and underparts are dingy white with sides of breast olive green and paler, yellowish-olive flanks. Legs are blue-gray. Bill is black with hook. Seasonal variations in adult plumage slight.

Adult Female (spring/summer)

Sexes are monomorphic in plumage and size although females tend to be slightly duller.

Juvenile

Similar to adults but upperparts brownish-gray; underparts entirely dull white, the flanks and undertail coverts faintly tinged yellowish.

General:

Medium sized Vireo. Length: 110-136mm. Wing: 21-25cm. Weight: 13-18 grams.

Behaviour:

Forages in a slow, deliberate manner, gleaning insects from foliage in upper parts of trees. Sometimes flies out to catch insects in midair or hovers to obtain insects from foliage tips. In summer it feeds almost entirely on insects and also eats some berries and small fruits, though especially in winter.

The conspicuous and continuous raspy up and down song of the Cassin’s Vireo sounds as if the bird is asking and answering its own questions. Its call consists of short harsh scolding notes.

Habitat:

Occupies coniferous, mixed coniferous/deciduous forests in the mountains and foothills. In coastal British Columbia it is most numerous in open stands of Douglas fir, oak and madrone on drier, rocky sites and in mixed woods of maple, alder, cottonwood, Pacific dogwood, Douglas fir and Western Red cedar.

Information:

The Cassin’s Vireo was named in honour of the nineteenth-century ornithologist, John Cassin, who published the first comprehensive work on the birds of western United States.

Three to five whitish eggs lightly spotted with brown and black are laid in a rather bulky open cup nest suspended by its rim, usually three to twelve feet above ground. The nest consists of grass, bark, plant fibers lined with plant down and hair.

Similar species: Blue-headed Vireo, Plumbeous Vireo, Bell’s Vireo, Hutton’s Vireo

Conservation Status:

Currently not listed as endangered, threatened or as a species of special concern by any state, provincial or federal wildlife agency.

Capture Rates

Cassin’s Vireo breed in the dry, open forests of mountains and foothills that are more indicative of the interior of BC. Therefore, Cassin’s Vireo is a rare visitor to Colony Farm as is reflected by the capture of only a few individuals.

Molt Summary:

PF: HY partial, PB: AHY complete; PA absent-partial.
The preformative usually includes all lesser, median and greater coverts, but no terts or rects.
The 1st PA includes 0-2 outer GCs and sometimes 1-3 terts, but no rects. (Adult PA similar).

Birds in juvenal plumage average browner and drabber. Juvenile M=F.

This ASY (sex unknown) wing is a prime example of what definitive adult plumage, with no discernible molt limits among the coverts or alula, looks like in spring.

Notice the broad, truncate primary coverts with distinct greenish-grey edging with no contrast in wear compared to the great coverts. There is also no contrast between the greater alula covert (A1) and lower alula feathers. Notice also the truncate visible remiges (primaries and secondaries), glossy black rachises and greenish edging to these feathers.