POLYMORPHISM IN THE ARCTIC SKUA STERCORARIUS PARASITICUS IN ICELAND

A survey of the frequency of the dark and light colour forms of the Arctic Skuas breeding in Iceland provided evidence of a south-north cline, the light form being more frequent in the north than in the south. In one area there were significantly more dark birds in 1971 than 30 years earlier, but in two other areas there was no evidence of a change in frequency. In northeast Iceland, where the light form is common, there was a statistical tendency towards unlike matings. It is suggested that the adaptive significance of the forms is to be sought in terms of facilitating departures from random mating, the genetic consequences of which are understood in general terms, although the ecological significance remains obscure.