Size: L: 115 cm. Wt: 2200-2600g. Males and females similar.
Breeding: upperparts slate blue; head white with black plumes; front of neck white with black streaks; bill yellow; legs and feet dark; plumes on neck and back.
Winter: plumes absent; bill yellower.
Juvenile: black crown with no plumes; upperparts less blue.
Nestling: semialtricial and downy; down on upperprats dark greyish-brown, except on crown, which is dark grey, and has long bristle tips that stand up straight; down on sides pale grey; down on underparts white; bill shorter and blunter than in adult.