Demography and selection in a population of Arctic Skuas

Abstract

From demographic data of the age distributions and reproductive rates of the colour phases of the Arctic Skua, estimates are obtained of the selective coefficients of the phases in males and females. There are two main components of the selection. Dark males are favoured by sexual selection: in new pairs mating with a particular female for the first time, they breed earlier in the breeding season than other males. This gives them a selective advantage because earlier pairs fledge more chicks than pairs breeding later in the season. Pale birds of both sexes are favoured because they start breeding at an earlier age than other phases and therefore have a greater chance of surviving to breed. The component of selection in favour of pale birds is considerably greater than the component of selection in favour of dark males. A computer model predicts that pale phases will gradually replace the darks. The gene frequency in the next generation agrees with the predicted frequency. The intrinsic rate of increase calculated for the population as a whole also predicts the actual increase in population size.