Montana Field Guide

Sexes monomorphic by plumage and size, although females tend to be slightly duller. Upperparts dull olive-green becoming brownish gray on crown and auriculars. Brownish-gray face contrasts with bold, white “spectacles,” formed by supraloral stripe and eye-ring, the latter interrupted by dusky loral streak. Wings and tail brownish black; 2 broad yellowish-white wing-bars. Remiges and rectrices finely edged olive-yellow or grayish olive; outer rectrix finely edged white. Rump brighter olive-green than mantle. Underparts dingy white with sides of breast smudged dull olive-green, and paler, yellowish-olive flanks. Iris brown; bill black with bluish-gray base; legs grayish blue. (Goguen, C. B., and D. R. Curson. 2002. Cassin’s Vireo (Vireo cassinii). In The Birds of North America, No. 615 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.)

Species tends to prefer dry, open forests. Occupies coniferous, mixed-coniferous/deciduous, and deciduous forests in mountains and foothills.

Diet consists almost exclusively of arthropods, spring through autumn. Winter diets consists of fleshy fruits.

Clutch size of three to five eggs (usually four). Incubation period 13-14 days.