Hybridisation between South polar skua (Catharacta maccormicki) and Brown skua (C. antarctica lonnbergi) in the antarctic Peninsula region

Hybridisation between South polar skua (C. maccormicki) and Brown skua (C. antarctica lonnbergi) in the area of the Antarctic Peninsula is known at least since the beginning of the last century but no survey has been done so far. This paper reviews information on the species composition of skua colonies of more than 10 pairs in the Antarctic Peninsula region, and the incidence of mixed pairs. Morphometrics, population size and breeding success were examined in detail at King George Island. The northward distribution of South polar skuas extended to King George Island (62 degree 11' S 59 degree 00' W), with a small outlying population on Signy Island (60 degree 45' S 45 degree 36' W), whereas Brown skuas did not breed further south than Anvers Island archipelago (64 degree 46' S 64 degree 03' W). The proportion of mixed pairs was highest at the northern end of the similar to 500-km-wide hybrid zone. Body size distribution of sympatric skuas from King George Island is clearly bimodal but overlaps considerably and hybrids cannot be identified. Skua population sizes at Potter Peninsula/King George Island remained stable between 1994 and 2004. Numbers of mixed breeding pairs fluctuated more strongly than those of pure species pairs. Breeding success of Brown skuas varied the least.