Montana Field Guide

Small to medium-sized warbler (10-13 cm in length). Male in alternate plumage is plain and lacks distinctive markings; the crown and nape are grayish, contrasting with bright olive green upperparts and whitish underparts. Distinct blackish eye-stripe and narrow white supercilium. Female in alternate plumage is similar but duller, with less contrast between the head and remaining upperparts, a less-defined eye-stripe and supercilium, and a yellowish wash across the breast and flanks. Basic-plumaged adults and immatures are similar: gray green above and underparts tinged yellow. Females are generally duller and more yellowish below than males. Immatures have more strongly washed yellow underparts than adults. (Rimmer and McFarland 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.

Distinguished from all other warbler species except Orange-crowned Warbler by overall plain plumage characterized by unmarked white to yellowish underparts, unmarked olive green upperparts, lack of distinct wing-bars or tail-spots, and dark eye-stripe below yellowish to whitish supercilium. Distinguished from Orange-crowned Warbler by white rather than yellow undertail-coverts; by lack of faint, blurred streaks on sides of breast; by richer, brighter green on mantle and scapulars; and by significantly shorter tail and generally longer supercilium. (Rimmer and McFarland 2012)

Nocturnal long-distance migrant; generally joins mixed-species foraging flocks, especially in fall (Rimmer and McFarland 2012).

Breeds in the boreal zone in deciduous, mixed, and coniferous forests; found in a variety of successional stages. Associated with open areas with grasses, dense shrubs, and scattered clumps of young deciduous trees; strongly associated with shrubs. (Rimmer and McFarland 2012)

Diet consists mainly of invertebrates; during the breeding season, lepidopteran caterpillars constitute the bulk of food volume. Well-documented specialist on spruce budworm caterpillars. (Rimmer and McFarland 2012)

Species exploits outbreaks of spruce budworm, and breeding densities fluctuate accordingly (Rimmer and McFarland 2012).

Nests are well hidden on the ground. Usually single-brooded, with 5-6 eggs per clutch. Little information, but incubation period about 11-12 days, at which time young leave the nest. (Rimmer and McFarland 2012)

There is no evidence that breeding habitat has been severely impacted, nor is there information on area sensitivity or fragmentation effects (Rimmer and McFarland 2012).