Long-term population size and trends of South Georgia Shags (Leucocarbo [atriceps] georgianus) at Signy Island, South Orkney Islands and Bird Island, South Georgia

The South Georgia Shag Leucocarbo [atriceps] georgianus has breeding populations on the islands of South Georgia, the South Sandwich and South Orkney Islands. The South Orkney Islands are estimated to support ~ 18%–37% of the global population and South Georgia a further 37%–69%. Here, we examine changes in South Georgia Shag population size and productivity from Signy Island, South Orkney Islands, over a 43 year period (1978/1979 to 2020/2021) and from Bird Island, South Georgia, over a 32 year period (1989/1990 to 2020/2021). Analysis of total nesting pairs at Signy Island revealed an overall decline of 40.9% (− 1.3% per annum), with an increase during the 1980s, followed by a fluctuating decline from the 1990s to 2020/2021. Although the two Signy Island colonies showed correlated fluctuations in numbers of nesting pairs, over the whole time period these colonies showed markedly different population trajectories, indicating the limitations of using part-island counts to infer whole island trends, particularly given the low breeding-site fidelity in this species. Nest occupation in the larger colony (596 nests in 1978/1979) declined by 77.2% (− 3.5% per annum) whilst the smaller colony (50 nests in 1978/1979) exhibited an increase of 492% (+ 3.8% per annum). A decline in occupied nests of 58.3%, (− 2.8% per annum), has occurred in the Bird Island population since 1989/1990. Continuation of the significant decline in breeding numbers revealed in this study may be of important conservation concern, particularly as this trend has been mirrored at another site within the South Orkneys