Size: L: 75 cm Wt: 1450-1850 g. Males are somewhat heavier than females. Seasonally dimorphic. This species requires four years to reach full adult plumage.
Breeding Adult: mantle and wings black, with obvious white leading edge on wings in flight; white primary tips small or absent due to feather wear; head and underparts white; tail white; bill yellow, with red spot; legs pale pink
Winter Adult: similar to breeding adult, but with slight dusky head streaks; extensive white primary tips
Juvenile: similar to first winter, but checkered pattern darker, neater; head and underparts darker, more uniformly streaked
Nestling: semiprecocial and downy; down long, soft; upperparts and throat grey, with small, blackish-brown spots and mottline on head, chin, throat, back, and rump; underparts whitish; bill dark blackish-purple; legs pinkish