Photo courtesy Charles Martinez
This week’s Bird of the Week, compliments of the Weminuche Audubon Society and Audubon Rockies, is the Cassin’s vireo.
This bird is one of our fall migrants, passing through on its journey from summer breeding grounds in dry, open forests of the far western parts of North America to its winter grounds in Mexico. In migration, it appears in almost any wooded habitat.
The name vireo is derived from the Latin word which translates as “to be green” and most birds in this family are colored in shades of olive green, gray and yellow. Prior to 1997, the Cassin’s was included with the blue-headed and plumbeous vireos in one species known as the solitary vireo. Genetic sequencing determined that they are in fact similar but separate species.
The plumbeous vireo, which spends the summer and breeds here, is a gray bird. The Cassin’s has olive-gray upper parts, a gray head, white underparts and a yellow wash on the sides and two white bars on the wings. Both of these vireos have prominent white eye rings that extend across the beak, giving them the appearance of wearing spectacles.
Like warblers, these vireos forage in trees for insects and spiders which they glean from needles, leaves, branches and twigs. However, unlike warblers which feed by flitting from place to place in the tree, vireos search slowly and methodically. Cassin’s vireos most often forage at fairly low levels in the outer sections of trees. In winter, they add small seeds and fruits to their diet.
John Cassin, for whom this bird is named, published the first major book on birds of the western United States.
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