Size: L: 40 cm. Wt: Males 700-750 g. Females 550 g. Males and females are dimorphic in size, but similar in colour. This species is not seasonally dimorphic.
Adult: body slate grey, darker above than below; head and neck black; outer undertail coverts white, inner ones black; bill whitish, with brown ring, and small, reddish-brown forehead shield; legs greenish-yellow, with long, lobed toes
Juvenile: similar to adult, but paler, with dusky head and neck, no forehead shield, and greyish legs
Nestling: precocial and downy; down thick, with long, fine, hair-like tips above; down black on head and back, greyish on underpartsdown of neck, throat and front of head with reddish to orange waxy tips; down of mantle and wings with yellow sheaths; crown bare, reddish; skin over eyes blue; bill reddish, with white tip; legs blackish