Enigmatic phylogeny of skuas: an alternative hypothesis

Last year, Cohen et al. presented molecular data suggesting the surprising result that both currently recognized genera of skuas, Stercorarius and Catharacta (Aves: Stercorariidae), are not monophyletic. However, the most enigmatic conclusion from their analysis, that S. pomarinus is sister to C. skua, rests solely on mtDNA sequence data. When the mtDNA data are analysed in a maximum likelihood framework that accounts for variation in evolutionary rates, Catharacta monophyly cannot be rejected. None of the best trees that can be derived from two nuclear data sets of Cohen et al. support the controversial pomarinus-C. skua node. We propose an alternative hypothesis, that pomarinus is sister to a monophyletic Catharacta, as the best explanation of the available molecular, morphological, and behavioural evidence.