Vancouver Avian Research Centre

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 Species: CASSIN’S VIREO  Vireo cassinii
   

 Description: 

The Cassin’s Vireo is a somewhat drab ‘spectacled’ bird of the West.  It is however conspicuous in the forest due to its tireless, raspy singing and harsh scolding call. 

The Cassin’s Vireo breeds from southern B.C. and southwest Alberta, western Montana, northern Idaho, Washington, along the west coast to southern California.  It winters in areas of southern California and Arizona and Mexico. 

Identification: 

General:  Medium sized Vireo with hooked bill and white ‘spectacles’. Length: 110-136mm.  Weight: 13-18g. 

Adult Male:  Upperparts are dull olive-green becoming gray on 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: 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. 

Similar Species:  Hutton’s Vireo is smaller with a narrower, incomplete eye-ring. Underparts are drab olive and there is less contrast in plumage.  Bell’s Vireo lacks full eye ring and has a whitish supercilium.  Warbling Vireo, though similar in size, has indistinct supercilium instead of eye ring. 

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 questions.  Its call consists of short harsh scolding notes. 

Habitat:  Occupies coniferous, mixed coniferous/deciduous and 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: 

This 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.

It was formerly included as a “Solitary Vireo”, with the Plumbeous and Blue-headed vireos however was recently split.  The Cassin’s Vireo includes two subspecies:  V.c. cassinii, which breeds along the west coast of North America and a non-migratory Mexican subspecies V.c. lucasanus found only in the far southern Cape District of Baja California Sur. 

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. 

Conservation: 

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. Therefore, Cassin's Vireo is a rare visitor to Colony Farm as is reflected by the capture of only a few individuals in April and zero capture rate for the remainder of the year. Capture rates are standardized as birds captured per 100 net hours from 2010 - 2012.

 

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