Family STERCORARIIDAE G.R. Gray, 1870

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Family STERCORARIIDAE G.R. Gray, 1870

Introduction

Members of the family Stercorariidae have traditionally been placed in two genera, Catharacta for larger species breeding mostly in high southern latitudes, and Stercorarius for smaller species breeding in high northern latitudes. Following various studies into Stercorariidae phylogeny (Cohen et al. 1997, Braun & Brumfield 1998, see summary in Christidis & Boles, 2008) this arrangement has been cast into doubt and Chu et al. (2009) suggest that a single genus should be recognised in Stercorariidae until further work can resolve relationships between the constituent species. Within Australia, five of the seven skua species visit on migration. Ten ultra taxa are currently recognisable, however relationships at the species level and below also remain in a state of flux (e.g. Ritz et al. 2009).

The stercorarids are marine species of large to moderate size - the larger birds are in the genus Catharacta. Catharacta antarctica often spends its winter to spring visiting Australia and Tasmania from its Antarctic and Subantarctic breeding grounds. The smaller, polytypic genus Stercorarius is more cosmopolitan, breeding in the Northern Hemisphere at higher latitudes and migrating to the Southern Hemisphere during the northern winter. Both are primarily piscivorous although invertebrates form part of their diet. They occur in offshore and pelagic waters sometimes entering bays, estuaries and harbours.

The large Catharacta species are a sombre brownish colour although paler shades occur due to wear of their plumage. Adornment of their wings consists of creamy white panels at the base of the primaries. The smaller Stercorarius apecies are variable, usually brownish above and paler below, the crown and face colour of members of this genus are generally darker. During breeding they have elongate central tail feathers. Their young, as with other Charadriiformes, are precocial, nidifugous and ptilopaedic.

The swift flight and robber tactics of Stercorariidae allows them to obtain much of their food through the pursuit of other seabirds, enticing them to drop their prey. The larger skuas also scavenge at the carcasses of beach-washed marine animals. They can be either single in occurrence or gregarious. Nests are constructed on open ground and are mostly slight depressions with little to no lining. Two oval to sub-elliptical eggs form a clutch; the eggs are similar in colour to those of the gulls, having an ochre-green base and being heavily blotched and spotted overall with darker browns and greens.