Size:  L: 45 cm Wt: Males: 420 g. Females: 390 g. Males and females are similar in plumage coloration. Seasonally dimorphic. This species requires two years to reach full adult plumage.

Winter Adult: mantle and wings grey, with black wing-tips forming neat black triangles in flight; head white, with greyish nape, and dusky smudge behind eye; underparts and tail white; bill yellow; legs black

First winter: mantle and scapulars grey; wings grey, with broad, black carpal bar, forming "W" across upper wings in flight; head white, with greyish nape, blackish half-collar on lower hindneck, and blackish spot behind eye; underparts white; tail white, with black terminal band; bill and legs black