Taxonomy
Skuas are related to gulls, waders, auks, and skimmers. In the three smaller species, all nesting exclusively in the Holarctic, breeding adults have the two central tail feathers obviously elongated, and at least some adults have white on the underparts and pale yellow on the neck. These characteristics are not shared by the larger species, all native to the Southern Hemisphere except for the Great Skua. Therefore the skuas are often split into two genera, with only the smaller species retained in Stercorarius, and the large species placed in Catharacta. However, based on genetics, behavior, and feather lice, the overall relationship among the species is best expressed by placing all in a single genus. The Pomarine and Great Skuas' mitochondrial DNA (inherited from the mother) is in fact more closely related to each other than it is to either Arctic or Long-tailed Skuas, or to the Southern Hemisphere species. Thus, hybridization must have played a considerable role in the evolution of the diversity of Northern Hemisphere skuas.
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