This species undergoes almost continual molt from spring through fall, and thus what is termed here as a ‘breeding plumage’ is merely the stage of molt that corresponds with the peak of the breeding season (May-June). Males in this plumage have buffy to greyish-buff upperparts (back, scapulars, rump, inner upperwing coverts) with fine, dark barring and vermiculations throughout. The wings (including most of the wing coverts) and tail are entirely white. The head, neck, upper breast, sides, and flanks are largely buffy or greyish-buff (throat, foreneck, breast, sides, and flanks usually with white background colour), with blackish and dark brown barring and mottling throughout (black usually forming bold, irregular splotches on the lower neck and breast); the remainder of the underparts (lower breast, belly, undertail coverts, feathered legs and feet) are white to pale grey. This briefly-held plumage is acquired gradually throughout the spring, and thus individuals observed during the April-May period are in a transitional plumage that is a patchwork of white and brown. These birds first begin to acquire dark-barred brown feathers on the head, neck, and upper breast, with feather acquisition then progressing onto the back, uppertail coverts, and finally the scapulars. A similar situation occurs following the peak of breeding activity, as males gradually acquire the white winter plumage between June and October, with some intermediate buffy-brown or cinnamon-buff feathers acquired for a brief period in mid-summer. These individuals first acquire white feathers on the belly, undertail coverts, and breast but retain finely vermiculated buffy-brown to grey-brown or olive-brown feathers on the upperparts, head, and neck throughout the late summer and early fall. The plumage appears greyer and duller in late summer and fall than in early summer (when it is buffier or slightly rufous-toned) due to feather wear and fading. The iris is dark and bill is blackish, and there are fleshy red combs above the eyes (particularly prominent at the height of breeding). The legs and feet are fully feathered with whitish feathers.
Like the male, the female undergoes almost continual molt between April and October/November. At the peak of the breeding season (May-June), the female is pale buffy or buffy-grey throughout with heavy dark brown and blackish barring on the head, neck, breast, sides, flanks, back, scapulars, and uppertail coverts. The wings and tail remain white throughout the summer, and the lower belly, undertail coverts, and feathered legs and feet are usually white or pale buff. Like males, birds in spring (April-May) gradually acquire the breeding plumage and generally appear as a patchwork of white and brown. Breeding-plumaged feathers are first acquired on the head, neck, and breast and subsequently on the upperparts (with the scapulars being the last feather tract replaced). Females also undergo a post-breeding molt (June-October) that is similar to the males, and are often indistinguishable from males in the late summer and early fall. Bare part colouration is similar to the male, but the red combs are less conspicuous.
This ‘winter’ plumage is acquired in October or November and is held until ~April of the following spring. The plumage of both sexes is entirely white, although there is sometimes the suggestion of very thin red combs in the male, particularly in late winter prior to courtship.
This plumage is held briefly during the summer of the first year, with an all-white winter plumage gradually acquired throughout the late summer and fall. Birds in this plumage are cinnamon-rufous to greyish-buff on the upperparts (including the wings and tail) and greyish-brown on the underparts, head, and neck, with heavy grey-brown, dark brown, and black mottling and barring throughout. The outermost two primaries are white in all but the youngest juveniles. The wings and tail gradually become white throughout the late summer and fall, followed by the remainder of the plumage in the fall.
Total Length: 31-32 cm
Mass: 325-490 g
Source: Braun et al. (1993); Sibley (2000)