This plumage is held between early spring (March) and mid-summer (July), and is acquired prior to spring migration. The upperparts are greyish-olive to olive-brown with indistinct darker streaks on the back and scapulars. The wings (including the upperwing coverts) are darker brown or blackish-brown with broad buffy-brown or olive-buff feather edges (sometimes giving the impression of two pale wing bars). The relatively long, square-tipped tail is dark brown to blackish-brown with paler olive-brown central feathers and extensive white in the outer feathers (most of the outermost feather, tip of the second-outermost feather). The underparts range from rich buff to relatively bright pinkish-buff, usually with variable fine, dusky-brown streaking on the breast and sides (absent in some individuals, particularly in the subspecies A.r.alticola). The head is greyish-olive to olive-brown (often slightly greyer in tone than the rest of the upperparts) with a pale buff to brighter pinkish-buff throat and relatively broad supercilium. The iris is dark, the relatively short, slender, pointed bill is dark brown to black (darkest on the culmen), and the legs and feet are blackish or dark grey (sometimes slightly paler and tinged with brown or pink).
This plumage is held between mid-summer (August) and late winter (February), and is acquired on the breeding grounds prior to fall migration. Non-breeding adults are similar in plumage to breeding-plumaged adults, but have darker olive-brown upperparts and paler buffy (sometimes nearly whitish) underparts with heavier and more prominent brown to blackish-brown streaking on the breast, sides, and flanks. The throat and supercilium are similarly paler buff to whitish-buff, with a prominent dark brown malar stripe separating the pale throat from the buffy moustachial area. The pale buffy wing bars are often more prominent in non-breeding plumage than they are in breeding plumage. Some non-breeding individuals can be unusually rich buff on the throat, supercilium, and underparts with relatively little dark streaking on the breast and sides. Bare part colouration is similar to that of the breeding adult, but the bill is typically paler and pinkish with a dark culmen and tip.
This plumage is held briefly during the late summer of the first year, but is lost in August or September prior to fall migration. It is similar to the non-breeding adult, but the upperparts have blacker and more extensive streaking as well as pale buffy feather edges (creating a scalloped appearance). In addition, the throat and breast are relatively heavily streaked with dark brown, with the streaking on the sides and flanks narrower and paler brown. The head pattern (particularly the supercilium) is less prominent than in the adult.
Total Length: 16-17 cm
Mass: 18.5-26 g
Source: Verbeek and Hendricks (1994); Sibley (2000)