ENIGMATIC PHYLOGENY OF SKUAS (AVES, STERCORARIIDAE)

Multiple sources of evidence show that the skuas (Aves: Stercorariidae ) are a monophyletic group, closely related to gulls (Laridae). On mor phological and behavioural evidence the Stercorariidae are divided int o two widely divergent genera, Catharacta and Stercorarius, consistent with observed levels of nuclear and mitochondrial gene divergence. Ca tharacta skuas are large-bodied and with one exception breed in the So uthern Hemisphere. Stercorarius skuas (otherwise known as jaegers) are smaller bodied and breed exclusively in the Northern Hemisphere. Evid ence from both mitochondrial and nuclear genomes and from ectoparasiti c lice (Insecta: Phthiraptera) shows that the Pomarine skua, S. pomari nus, which has been recognized as being somewhat intermediate in certa in morphological and behavioural characteristics, is much more closely related to species in the genus Catharacta, especially to the Norther n Hemisphere-breeding Great skua, C. skua, than it is to the other two Stercorarius skuas, the Arctic skua, S. parasiticus and the Longtaile d skua, S. longicaudus. Three possible explanations that might account for this discordant aspect of skua phylogeny are explored. These invo lve (i) the segregation of ancestral polymorphism, (ii) convergent evo lution of morphology and behaviour or (iii) inter-generic hybridizatio n. The available evidence from both nuclear and mitochondrial genomes does not exclude any of these hypotheses. Thus, resolution of this eni gma of skua phylogeny awaits further work.