Mating preferences and sexual selection in the Arctic Skua

Summary

Data of the breeding of pale, intermediate and dark phenotypes in a population of Arctic Skuas are analysed and fitted to models of sexual selection. In the model which fits best, groups of females become ready to breed during successive intervals in the breeding season. Females with preferences mate first, some preferring the dark males and others the intermediates; the remaining females then mate at random. At maximum likelihood, 14 per cent of females prefer dark males and 29 per cent prefer intermediate males. The likelihood surfaces of the models are illustrated with respect to the parameters of the mating preferences. An analysis of X² shows that after fitting the mating preferences and allowing for variation in breeding times, no heterogeneity is left in the data. Observations of assortative mating and the behaviour during pairing also support the model. The female preferences can be explained as the result of different thresholds in the females' responses. These may have evolved in earlier, warmer periods when dark and intermediate phenotypes must have had an overall advantage.