Abundance and spatial distribution of sympatrically breeding Catharacta spp. (skuas) in Admiralty Bay, King George Island, Antarctica

We examined the abundance and spatial distribution of sympatrically breeding skuas (Catharacta spp.) within Admiralty Bay, King George Island, Antarctica, during the austral summer of 2004/2005 in relation to spatial variables, which correspond to access to resources and nesting site safety and quality. We also compared the distribution and abundance of skua pairs observed in 2004/2005 to published skua census data from 1978/1979. Similar to previous studies, we found that brown skua (C. antarctica lonnbergi) pairs often nested in close proximity to penguin colonies and actively excluded other pair types from having direct access to penguin resources. In areas directly around penguin colonies, brown skua displace south polar skua (C. maccormicki) and other pair types, indirectly forcing them to nest in possibly lower quality territories, which are farther away from the coastline and in areas with lower incident solar radiation. When examining skua population trends, we discovered that the total number of breeding skuas in Admiralty Bay had increased by 293%, from 128 to 468 pairs, since 1978/1979. This dramatic increase was driven primarily by a tenfold increase in south polar skua pairs, as well as smaller increases in mixed and hybrid pairs. In contrast, there has been an overall decline (by 40%) in brown skua pairs during this same time, driven primarily by a large decrease in the breeding density of brown skua pairs in areas without penguin colonies.