POLYMORPHISM, MATING PREFERENCES AND SEXUAL SELECTION IN THE ARCTIC SKUA

The intensive study of the Arctic skua (Stercorarius parasiticus) popu lation on Fair Isle has provided an example of the maintenance of a st able polymorphism by sexual selection in conjunction with assortative mating (O'Donald, 1983). We tested several of O'Donald's hypotheses us ing data collected from the large colony of Arctic skuas on Foula. The re was some evidence for assortative mating of the phenotypes but this did not confer any short-term fitness advantage. In contrast to the s ituation on Fair Isle, we found little indication that sexual selectio n took place for dark males since no differences in timing of breeding or productivity were evident between the two male phenotypes in new p artnerships. Nor could we detect variation in adult body size, body co ndition, or any life-history characteristic between melanic and pale b irds of either sex. Many of the selection pressures documented on Fair Isle appear to be absent at the neighbouring Foula, despite a fair de gree of gene flow between these colonies. The frequency of pale adults on Foula and in Shetland as a whole has declined since the 1970s, ind icating that currently directional selection may be favouring the mela nic phenotypes at the southern end of the breeding range.