Bird of the Month: May 2002

Cassin’s Vireo

Vireo cassinii

by David Fix

It’s 93˚ in the shade, right around the optimum operating temperature for flies. Morning’s marine layer is forgotten. Vivid heat haze softens even the nearer ridges; annual cicadas ziznzik everywhere in the oaks and scattered overstory firs: it’s mid-July east of the Redwood Curtain now. The slumberous two-o’clocks cause human activity to wane, and many woodland birds to cease singing, ‘closet-up’, and while away an hour preening.

This lull of early afternoon in the foothills passes typically unnoticed by Cassin’s Vireos. Throughout their April-September period of visitation in northwestern California, the songs of territorial males carry through the timber and across the vest-pocket meadows and glades within nearly all forests of any age in our region, except for the immediate coastal lowlands. Should a raptor, human, or some other persistent disturbance in the woods result in a frenzy of anxious birds, one or two Cassin’s Vireos will usually be among the mob. This is one of the more routine birds of mature and old-growth conifer stands supporting at least a minimal hardwood component, and is commonly encountered in nearly every closed-canopy situation dominated by oaks. The 1995-1999 Humboldt County Breeding Bird Atlas mapped detections throughout most of the county. Breeding confirmations—often based on adults carrying food—were frequent. Despite its abundance over much of temperate western North America, Cassin’s Vireo is nevertheless a species known well only by those who seek birds just to look at them. Persons who feed sparrows all winter may remain unaware that a pair of these vireos nests in the woodlot down the street each summer.

The bird we now perceive as Cassin’s Vireo was long known by that common name during a previous birding era. However, it was then classified as merely a western subspecies of what was until a few years ago the ‘Solitary Vireo’. This complex involves several sibling species varying somewhat in coloration, size, song, and—particularly in the breeding season--habitat preference. Cassin’s Vireo is the bird breeding from southwestern Canada southward through all but the driest portions of the Pacific States, and wintering in Middle America. In migration, they appear in the Great Basin and in the Rockies. Cassin’s Vireos usually appear in early April, but don’t become common until well into the month. Singing spring migrants outside appropriate nesting habitat are not common coastward. These vireos are noted for their habit of suspending their woven, semi-pendulant nest in a twig crotch scarcely above head height. Lowermost limbs of tall trees and the tangled canopies of shrub-trees are most often used. From 3-5 (usually 4) dark-spotted white eggs are incubated for about two weeks, with nestlings fed caterpillars and other such prey until they fledge in another half-month. A smattering of birds (which are generally quite plain, and are likely mostly juveniles) occurs in alder stands and similar unmanaged taller vegetation from late August into early October, occasionally up to the time of the Christmas Bird Counts.

As a group, ‘Solitary’ Vireos of whatever race are characterized by mid-sized, Chevy-standard proportions, a stout hooked bill, thin whitish ‘spectacles’, and two pale wing-bars and thin pale tertial edgings that contrast with a darker wing panel. Songs of all forms are similar: an insistent if deliberate progression of unmusical phrases, each of which seems incongruous with the preceding phrase: See-Me?. . . De-troit . . . sur-Real! Their call-note is a peevish scold ordinarily of one syllable, but variable.