Size: L: 55 cm Wt: 600 g. Males and females are similar. This species is seasonally dimorphic, and takes four years to attain adult plumage.
Winter Adult: mantle and wings dark grey, with black wing-tips and white primary tips; head heavily streaked with brown; underparts mostly white, with some pale spotted on breast; tail white; bill yellow, with black and red spots; legs greenish-yellow
Third Winter: similar to adult, but wings and tail with some brown spotting, and bill more heavily smudged with black
Second Winter: mantle grey; wings brown, with blackish-brown tips and no white primary tips; head whitish, with faint speckling; underparts with heavy, dusky spots and streaks; tail blackish; bill flesh-coloured, with black tip; legs grey
First Winter: mantle and wings brown, with blackish-brown tips, and no white primary tips; head and underparts brown, with heavy, dusky streaking and spotting; tail blackish; bill flesh-coloured, with black tip; legs pinkish