Phylogeny, behaviour, plumage evolution and neoteny in skuas Stercorariidae

The evolution of skuas is puzzling in several respects. Based on cladistic analysis of behaviour and review of other evidence, a likely phylogeny is suggested which is used to trace the evolution of plumage coloration and other trails in skuas. Behavioural, parasitological and four kinds of genetic evidence now show that the Pomarine Skua Stercorarius pomarinus is more closely related to great skuas, usually placed in the genus Catharacta, than to the two smaller Stercorarius species. All skuas should therefore be placed in the same genus, Stercorarius. Alternatively, the generic name of pomarinus should be changed to Catharacta. High similarity in mtDNA and feather lice (Phtiraptera) between Pomarine and North Atlantic Great Skua Catharacta skua is suggested to be explained by hybridization between them. Plumage coloration among skuas is puzzling, as the adult plumage of great skuas differs much from that of other skuas. This is related to a less pronounced change from juvenile to adult breeding plumage in great skuas, which are here suggested to be neotenic in several respects. Possible reasons for these differences between great and other skuas are suggested.