Vireos are excellent little creatures that are so akin to twigs and leaves it seems they may have coevolved with the trees themselves.
|A freshly molted Cassin's Vireo (L), especially a male, closely resembles a Blue-headed Vireo. A worn Cassin's (center) most closely resembles a Plumbeous Vireo (R). © Keith Hansen.|
They are mostly green and yellow and have developed the craft of "hover-gleaning" to a fine-tuned foraging strategy. Hover-gleaning enables these insectivores to harvest insects and arachnids and their eggs from the ventral side of leaves.
Like hummingbirds and wood-warblers, vireos are only found in the Americas. There are 40 species in this tropical family, and only 15 have been documented in the United States.
In the 41st supplement to the AOU Checklist1, the American Ornithologists' Union Committee on Taxonomy and Nomenclature rearranged the phylogenetic order of birds and relocated vireos–from just before wood-warblers back to a rank between shrikes and corvids (ravens, crows, and jays).
|Blue-headed Vireo. Note the sharp definition between cheek and throat. On Cassin's, the line is blurred. © Keith Hansen.|
While that came as a surprise to some, anyone who knew the birds knew that similarities between warblers and vireos were superficial (that they were all small and mostly green birds that sang a lot and foraged in the leaves of trees.) Those familiar with the birds also knew that, in addition to structural differences, vireos have more complicated personalities than do warblers. While wood-warblers show few behavioral ups and downs, any vireo can be demure then sassy, placid then pugnacious. In the hand, warblers lie docile. Vireos fight, complain, and may chew upon a human knuckle.
Three species split from one
At the same time the AOU committee repositioned vireos in birds' taxonomic order, they split what had always been known as Solitary Vireo into three species: Blue-headed (Eastern); Cassin's (West Coast); and Plumbeous (arid Southwest mountains). This, too, came as no surprise, as these taxa2 are so closely related. Grounds for their assignment to species rather than subspecies level comes down to implications gathered from field studies. (See note at end of article.)
Plumbeous Vireo is the palest of the three, seldom showing any yellow or green. It is remarkable for the uniformity of pale gray coloring on its crown, back, and cheeks.
Only rarely do the most "worn" Cassin's show such uniformly patterned upperparts. Pale Cassin's, though—especially in spring and summer—may appear mostly gray above and white below, much like Plumbeous. Tinges of yellow on the lower sides, and a greenish panel on the closed wings (the narrow, leading edge of each remige, or flight feather, all stacked up), may be all that betrays the identity of a true Cassin's, as distinct from a Plumbeous.
On the other end of the cline, Blue-headed Vireos are the most colorful. Some are resplendent with their blue crowns, lime-green backs, lemon-yellow sides, and fancy white throats and bellies. A bright Cassin's male comes close to being as colorful as a ‘dull' Blue-headed, and the most consistent character differentiating the two may be in the sharpness of the line that separates the white throat from the blue-gray face (auriculars)–crisp and sharp on Blue-headed, blurrier on Cassin's. Most identifications will be easy.
|Hutton's Vireo. Plumage color and unique calls and song distinguish it from the "Solitary" types. © Keith Hansen.|
While similar to these three species in size and proportions, Hutton's Vireo would rarely pose a problem of identification. Hutton's are overall more olive-green and dull yellow, and their wingbars and spectacles aren't nearly as sharp and contrasty. Still, we have seen greenish-crowned Cassin's called Hutton's, and fresh Hutton's called Cassin's.
Nests and songs
Each of these birds suspends its finely woven nest from a forked branch, at or near the outer edge of a bush or small tree.
All (except Hutton's) have musically similar songs that (anthropomorphizing) may seem as if the bird proposes a question, thinks a moment, then answers the question: "doodlelee?—sheerio!"
In today's world, it is unfortunate for vireos that both parents share incubation duties and often sing from the nest, as that behavior allows cowbirds to home in more easily on cozy places to dump their eggs.3
Vireonidae is my favorite family of North American passerines (although Mimidae, the thrashers, is a really close second). Vireos are assertive, spunky, lovely, good vocalists… and some are hard to see, even when close! Many species are rare nearly everywhere, and of the vireos formerly known as Solitary, the Plumbeous and Blue-headed both appear nearly every year as vagrants on the mid-California coast—about one each! Seeking and finding them adds greatly to the joys of witnessing migration.