Credit: FRANK LANE/E. & D.HOSKING

In Europe, the ruddy duck (Oxyura jamaicensis, right) was once seen as a welcome North American addition to wildfowl collections. But the species escaped, became widespread and has threatened the existence of the native white-headed duck, O. leucocephalus, by mating with the females. In Britain, the reluctant decision was taken to control ruddy ducks by shooting them.

But why should the bigger female white-headed ducks prefer the smaller ruddy-duck drakes rather than males of their own species? A clue comes from a paper by K. G. McCracken in Auk (117, 820–825; 2000). Studies of the related Argentine lake duck O. vittata reveal that the male has an especially large penis — 20 cm, when erect, which is about half of its owner's length. McCracken includes observations of drake ruddy ducks displaying their penises, apparently to impress the females; similar displays, where the males compete in groups for the favours of the females, occur in all the six other species of the Oxyura genus.

Whether penis size is an explanation for why female white-headed ducks preferentially mate with male ruddies remains a matter of speculation, however. Further anatomical and behavioural studies — a Kinsey report for ducks, as it were — are needed to resolve the question.

On the conservation front, the spread of ruddy ducks to Spain and Turkey is threatening the white-headed populations there. In Spain, the interloper and any hybrids (which are fertile) are shot in an attempt to allow pure-breeding of O. leucocephalus. An encouraging report comes from one site near Alicante, on the Mediterranean coast. The estimated population in August this year was over 4,000, compared with 22 in the 1970s.