Eiders are a large, heavily built, duck with a distinctive wedge shaped beak and long sloping forehead
(Eider head details taken from images by Tom Cadwallender. Note the wedge-shaped bill and sloping forehead)
Males in particular are easily spotted with their white and black colouring, black cap and greenish neck markings. They also have a distinctive cooing call.
Female eiders are a reddish-brown colour with distinctive black barring on the feathers.