A medium sized diving duck. Males look black and white with a greenish black head and a circular white patch in front of the yellow eye. Females are smaller, and are mottled grey with a chocolate brown head. In flight, birds show a large area of white on the inner wing. First nested in Scotland in 1970, and since then birds have been attracted to nest in specially designed boxes put up on trees close to water. In winter, birds from Northern Europe visit the UK.