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.

Many seabirds are characterised by long life spans and delayed maturity, and spread their reproduction across many years. This is particularly true for the most numerous species breeding at higher latitudes, indicating this way of life has proven successful over the longer term in variable environments where food is not always plentiful close to the colonies. Such a life history does, however, also make the populations very sensitive to changes in adult survival (e.g. Cairns 1987, 1992).

The Atlantic Puffin Fratercula arctica (hereafter ‘Puffin’), is a typical species in this context, laying single-egg clutches only and rarely breeding before five years old (e.g. Harris & Wanless 2011). Although probably still the most numerous seabird in the northeast Atlantic (e.g. Barrett et al. 2006; Mitchell et al. 2004), the Puffin is now rated as Vulnerable on the IUCN Red List of Threatened Species (BirdLife International 2017) following decades of substantial population decrease in several of its historical strongholds (e.g. Harris & Wanless 2011). Many of these negative trends may be explained by long-term recruitment failures (e.g. Durant et al. 2003; Erpur Snær Hansen pers. comm.) and no big differences in adult survival rates among colonies have yet been documented (Harris et al. 2005). Nevertheless, occasional events of reduced survival may have important effects on any seabird population (e.g. Frederiksen et al. 2008), warranting that mass mortality events should be examined to uncover their extents and causes, and to identify what populations, age classes and sexes are affected.

A more pelagic and dispersed distribution of Puffins outside the breeding season may explain why they are less numerous victims of coastal oil spills than for instance Common Guillemots Uria aalge (e.g. ICES 2005). Incidents involving substantial numbers of emaciated Puffins washed up dead on beaches have, however, been registered in most parts of the European distribution range, although most often in UK waters where beached birds surveys (BBS) are most extensive. Harris & Wanless (2011) list 23 wrecks of Puffins in the east Atlantic since 1856, of which 12 occurred in the last 40 years (i.e. the period when Puffin population trends have been monitored with reasonable accuracy) and five in the last ten years (2002–2011) covered by that review. Subsequently, there was an extensive wreck in the Bay of Biscay and southwestern UK waters in midwinter 2013/2014 (Morley et al. 2016), and two episodes in southwest Norway; one in February 2003 (Aarvak & Anker-Nilssen 2005) that overlapped in time with the one reported from the southern North Sea (Camphuysen 2003), and one in February–March 2016 (this study). With one exception from Norfolk in May 1856 (see Harris & Wanless 2011), all timed reports of wrecks involving significant numbers of Puffins are from outside the breeding season, and we are not aware of any similar incidents in the west Atlantic where Puffins are much less numerous.

Here we report results of investigations carried out following an extensive wreck of Puffins recorded on beaches in Vest-Agder and Rogaland counties in southwest Norway in late winter 2015/16. These areas have been subject to regular BBS since the 1990s, enabling us to evaluate the relative magnitude of the event. Our primary aims are to shed light on possible reasons for the wreck, document the age and sex distribution and body condition of the birds that were hit, and uncover the breeding areas from which they most likely originated.

This study was financed by the Norwegian Environment Agency who also authorized the collection of birds for post-mortem examinations. We are especially indebted to Tor Oddvar Hansen for collecting most of the birds from the Lista area, as well as providing suitable facilities for and assisting ML in the post-mortem examinations of birds at the Lista Nature Museum. Similarly, we thank the Rogaland division of the Norwegian Ornithological Society and their leader Steinar Eldøy for organising a parallel collection of birds on beaches there and for their long-time commitment in running the local BBS programme. Arne Follestad kindly brought the carcasses from Rogaland (as cabin luggage!) to NINA in Trondheim, where ML examined them with devoted assistance of Rakel J. Alvestad, Katrine Andresen, Vegard Sandøy Bråthen, Vilde Haukenes, Tanja Kleeb and Sandra de Rijcke. We also thank Knut Madslien at the Norwegian Veterinary Institute in Oslo who conducted an autopsy of three birds and tested them for avian influenza. The bird ringing centres in UK and Norway and the EURING Data Bank provided us with details of all relevant recoveries of ringed Puffins. We thank Mark Newell and the many people who collected field data on colour-ringed Puffins on the Isle of May over many years, and Scottish Natural Heritage for allowing us to work on the Isle of May National Nature Reserve. The fieldwork on the Isle of May was funded by the Natural Environment Research Council and the Joint Nature Conservation Committee’s Integrated Seabird Monitoring Programme. Brett K. Sandercock gave valuable comments to the interpretation of the survival analysis.

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