MacGillivray's Warbler

Overview

The MacGillivray's Warbler is a secretive, ground-hugging warbler of dense shrub thickets across mountainous and lowland habitats of western North America. It closely resembles the eastern Mourning Warbler in overall pattern, with a gray hood, dark throat markings in males, and bright yellow underparts, but is reliably separated by the presence of broken white crescents around the eye.

This species favors dense riparian and montane shrub habitat, often along streams or in regenerating burned or logged areas, where it stays low and difficult to observe for long.

Feather Identification

• Eye-ring: The most reliable feature is a pair of broken white crescents, one above and one below the eye, clearly interrupting the gray hood feathers; this feature is absent in the similar Mourning Warbler.
• Hood: Adult feathers form a gray hood over the head and upper breast, with adult males showing a dark, sometimes blackish bib across the lower throat.
• Underparts: Breast, belly, and undertail feathers are bright yellow, contrasting with the gray hood.
• Upperparts: Back, wing, and tail feathers are plain olive-green without wing bars.
• Female/immature differences: Females and immatures show a paler gray hood and a less developed or absent dark bib, though the broken white eye crescents are usually still detectable, even if less bold than in adult males.

Plumage Details

Adult males display the boldest gray hood, the most distinct dark bib across the throat, and the most obvious white eye crescents. Females and immatures show a paler, more diffuse gray wash on the head and breast, a reduced or absent dark throat patch, and somewhat less prominent eye crescents, though these remain a useful distinguishing feature at most ages.

A complete molt occurs on the breeding grounds in late summer, with plumage remaining relatively stable through migration and winter; feather wear over the season can make the white eye crescents appear slightly less crisp late in the breeding period.

Habitat & Range

MacGillivray's Warblers breed in dense shrubby vegetation across much of the mountainous and lowland West, from southeastern Alaska and western Canada south through the Rocky Mountains and Pacific states, favoring streamside thickets, regenerating burns, and shrubby forest edges, often at moderate to high elevations.

This species is a medium- to long-distance migrant, wintering from Mexico south through Central America, generally in similarly dense shrub and forest-edge habitats at a range of elevations.

Behavior & Field Notes

This warbler forages low within dense shrub tangles, gleaning insects and other invertebrates from foliage and occasionally the ground, rarely venturing into open habitat. Its secretive nature means it is frequently detected by song well before it comes into view.

The nest is a bulky cup built low in dense shrubs, often within a few feet of the ground near water. The song is a rolling series of paired notes similar to the Mourning Warbler's but often described as slightly more variable in ending, and the two species' ranges overlap only narrowly, making the eye-ring the most dependable feature where both might occur.