Resurvey reveals arrested population growth of the largest UK colony of European Storm-petrels Hydrobates pelagicus, Mousa, Shetland

Abstract

The typical life history of long-lived seabirds makes their populations extra sensitive to decreases in adult survival. It is therefore important to uncover the extent, causes and consequences of any incident that involves massive die-offs of such species. Towards the end of a stormy winter, large numbers of dead Atlantic Puffins Fratercula arctica were beached in Vest-Agder and Rogaland counties, southwest Norway in February and March 2016. There were also reports of smaller numbers of birds found further north along the Norwegian coast, but it was not possible to assess the total extent of the wreck directly. The first birds stranded on 31 January, the day after the storm Gertrude (in Norway named Tor) caused the strongest winds ever recorded in Norway. Post-mortem examinations of 200 carcasses collected in the two counties over the following two months indicated that 59% of the birds were adults, whereas 9% were first-winter birds. Virological examination of three birds tested negative for Influenza A. All birds were emaciated and lacked significant fat deposits, and had presumably starved to death. Most birds, especially among adults, were in the last stage of primary moult, and had probably been flightless only a few weeks earlier. This may help explain why the different age groups were so disproportionately affected. The mean wing length of adults not in active moult indicated the birds originated mainly from colonies on the east coast of the UK. This conclusion was in accordance with the origins of eight ringed birds found dead in connection with the incident, seven of which were adults with a mean age of at least 21.6 years. It also fits well with results from recent tracking studies of birds with geolocators. The apparent survival of adult birds breeding on the Isle of May, southeast Scotland, was much lower between 2015 and 2016 than in most other years, which, unlike what has been possible for many other incidents, demonstrates that this wreck probably had a serious impact on the population level.