Montana Field Guide

Small passerine: total length 124 to 138 mm, mass 12 to 20 g. Upperparts smooth neutral gray; rump tinged olive green. Underparts near-white; sides of breast smudged pale grayish olive; flanks very pale sulfur yellow in some individuals and dull white in others. Head has broad white supraloral stripe and eye-ring, the latter interrupted by dusky loral streak. Wings and tail blackish neutral gray, with 2 broad wing-bars; outer rectrix broadly edged white. Remiges and rectrices edged pale olive gray (appearing near-white in the field) or rarely, olive yellow. Iris brown; bill black with bluish gray base; legs grayish blue. Sexes monomorphic by plumage and size. Female distinguished by a vascularized brood patch. (Goguen and Curson 2012)

For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.

Most similar to Cassin’s Vireo, which is slightly smaller, greener on the back, slightly browner gray on the head, yellowish white wing-bars and undertail-coverts, and brighter, more extensively yellow flanks. The two species are not always distinguishable in the field. The Plumbeous Vireo also is similar to an accidental in Montana, the Blue-headed Vireo, which is slightly smaller and distinctly more colorful (bright olive green on the black with sulfur yellow flanks and yellowish undertail-coverts). (Goguen and Curson 2012)

Migratory in the United States, but resident to the south. Generally solitary in migration. (Goguen and Curson 2012)

Breeds in warm, dry montane forests of pine, oak and juniper. Typical habitat in the U.S. is dominated by ponderosa pine. (Goguen and Curson 2012)

Diet is almost exclusively arthropods, spring through fall. Fruit and other plant materials may be important during winter. (Goguen and Curson 2012)

Males defend breeding territories. Territory size is not dependent on population density; much apparently suitable habitat is often unoccupied. (Goguen and Curson 2012)

Nest is suspended by its upper rim from twigs, often toward end of live branch. Eggs are oval, white to creamy white, sparingly marked. Clutch size is usually 4. Arrival dates early to mid-May at northern and higher-elevation sites. (Goguen and Curson 2012)

Within the breeding range, the greatest threat to the Plumbeous Vireo seems to be degradation of habitat quality via the spread of Brown-headed Cowbirds. Nest parasitism has been shown to increase in proximity to livestock grazing, roads, forest openings or edges, and residential/urban areas. (Goguen and Curson 2012)