Skeletal morphology and the phylogeny of skuas (Aves: Charadriiformes,Stercorariidae)

The skuas (Aves: Charadriiformes, Stercorariidae) consist of two assemblages. On the basis of size, plumage, and distributional similarities, each of the two assemblages has long been considered monophyletic, and this traditional hypothesis has commonly been manifested in the recognition of two genera, Stercorarius and Catharacta; conversely, more recently collected molecular and ectoparasite evidence yields an alternative hypothesis, in which one member of Stercorarius, Stercorarius pomarinus, is more closely related to the forms in Catharacta than to the other Stercorarius sp. In this study we used skeletal morphology to test the competing hypotheses of skua phylogeny. Cladistic analysis of 141 osteological characters provided strong support for the molecular/ectoparasite hypothesis. However, those skeletal data did not support a sister‐taxon relationship between S. pomarinus and Catharacta skua, as inferred from mitochondrial DNA sequence data; instead, they resolved pomarinus as the sister of a monophyletic Catharacta. Additionally, our skeletal evidence did not support a sister‐group relationship between skuas and auks, as constraining skua/auk monophyly increased the tree length by nearly 5%. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 157 , 612–621.