An abnormal movement of auks occurred in the eastern Skagerrak in the third week of September 2007. Large numbers of Razorbills Alca torda were reported along the coasts of southeast Norway and western Sweden, many thousands entered Oslofjorden (Norway), and their migration past the northern tip of Denmark into the Kattegat began a month earlier than normal. This preceded heavy mortality of the species that lasted several weeks, and numbered thousands of individuals. Unusually for the time of year, Razorbills greatly outnumbered Common Guillemots Uria aalge in reports of live and dead birds. Of 376 Razorbills collected in Oslofjorden, 87% were adults, 9% immatures, and 4% juveniles. Among 326 adults, females (71%) outnumbered males, and 18% showed two white inner bill grooves instead of the normal one. All birds were extremely emaciated and had presumably starved to death. Virtually all adults and older immatures were still regrowing their outer primaries after the post-breeding moult, whereas those of juveniles were fully grown. Most, if not all, belonged to A. t. islandica populations breeding in the British Isles, Faroes or Iceland, and few, if any, were from A. t. torda populations of the Baltic, Norway or Russia; the 23 ringed birds found in the Skagerrak and Kattegat, mostly adults, all came from Scottish colonies. Population effects at these colonies were not obvious, but adult survival in 2007–08 was low at one colony in eastern Scotland. Long-term beached bird data indicated that while not on the scale of that in the Skagerrak and Kattegat, Razorbill mortality was abnormally high over a wide area of the North Sea in autumn 2007. The age and sex structure of the mortality and its possible causes are discussed.

The Razorbill Alca torda is the least abundant pelagic auk in the Atlantic (Merne & Mitchell 2004). Two subspecies are recognised (Vaurie 1965; Cramp 1985). In the northeast Atlantic, A. t. torda breeds from the Baltic Sea along the Norwegian coast to Jan Mayen, Svalbard and Russia, in total 39,000–62,000 pairs, while in the northwest Atlantic, 40,000–43,000 pairs breed from Greenland south through eastern Canada to Maine, USA. All 530,000 pairs of A. t. islandica breed in northwest Europe: in France (25 pairs), the British Isles (145,000), Faroes (4,500) and Iceland (380,000). In total, 94% of the world population of 610,000–630,000 pairs breed in the northeast Atlantic (Merne & Mitchell 2004; Barrett et al. 2006).

There is a strong clinal increase in the size (wing length and bill depth at gonys) of breeding birds in the northeast Atlantic from southwest to northeast, most strongly correlated with geographical position (coarsely indexed as latitude + longitude) and, to a lesser degree, negatively correlated with sea surface temper- atures in the colony area (Barrett et al. 1997). Razorbills have a semi-precocial fledging strategy, the chick leaving the colony at 16–20 days old and 25% of adult body weight (Gaston 1985). It is accompanied by the male parent (Wanless & Harris 1986), which undergoes a full moult of contour and flight feathers at this time; the duration of this period of dependence is uncertain but lasts ‘several weeks’, or longer (Harris & Birkhead 1985; Jones & Rees 1985). Females continue to visit the breeding site after the male has gone, but then undergo the same moult as males.

The Skagerrak and Kattegat are important wintering areas for Razorbills (Skov et al. 1995; Petersen & Nielsen 2011). Birds in the Baltic population are relatively sedentary although a few may move into the Skagerrak and eastern North Sea in winter. Many birds from Russia and northern Norway winter off southern Norway, while some of the large Icelandic population winters in the North Sea (Hudson & Mead 1984; Lyngs & Kampp 1996; Bakken et al. 2003; Bønløkke et al. 2006; Bakken & Anker-Nilssen in press). Little is known of winter movements of Faroese birds (J.- K. Jensen pers. comm.). Some Razorbills from southwest Britain and Ireland may winter in the North Sea, but these will be greatly outnumbered by birds from eastern England and Scotland (Merne 2002). Migratory distance and the constraint of the post-breeding moult mean birds from different populations may arrive in the eastern North Sea, the Skagerrak and Kattegat at different times of autumn, while successful males, which must accompany the growing young at sea, may arrive later than females, failed breeders and immatures.

In the third week of September 2007, unusual numbers of Razorbills were observed flying into Oslofjorden, Norway. Over the following two weeks, many hundreds of dead and moribund auks were found along the coast of southeast Norway and western Sweden, some also being reported inland (Isaksen & Bredesen 2007). Most were Razorbills, but Common Guillemots Uria aalge and a few Atlantic Puffins Fratercula arctica were also found. A sample of dead Razorbills from the Oslofjorden area was stored frozen for later post-mortem examination, while a small sample was also collected at Skagen, Denmark.

We describe the development and extent of the incident using sea-watching and beached bird survey data, and report on post-mortem examinations which provide information on body condition, the probable cause(s) of death, and the age and sex structure of the mortality. Heavy mortality of adults is more likely to have population effects than that of young birds, whose mortality in the first year of life is higher than that of adults (Lyngs 1994; Harris et al. 2000). A heavily biased sex ratio in the mortality, especially of adults in monogamous species such as auks, may also be more likely to result in population effects than a balanced one. We also present data on biometrics and ringing recoveries of Razorbills to help indicate breeding origins. Finally, we discuss possible reasons for the incident, and assess its effects at the population level.

We thank the Agency for Urban Environment, City of Oslo, for the collection and storage of birds, and Morten Bergan, Bård Bredesen, Bjørn Aksel Bjerke, Jan Erik Røer and Tormod Burkey for helping collect dead birds. Norwegian post-mortem studies were carried out at the Norwegian Institute for Nature Research (NINA) with financial support from the Norwegian Directorate for Nature Management. Natural History Museum of Oslo (Jan T. Lifjeld and colleagues) provided storage, genetic sampling and transport facilities, and Seblewengel Bekele Talle provided lab assistance. Kjell Handeland, Norwegian Veterinary Institute performed veterinary examinations. Egil Soglo permitted use of count data from Møringa organised by TønsbergFugl (http://home.online.no/~sve-a4/; data currently being transferred to www.artsobservasjoner.no). Sander Fiskvik, Signe Christensen-Dalsgaard and Jorunn Mittet Eriksen assisted during the post-mortem examinations at NINA. Beached bird survey data were provided by David Fleet and Martin Schulze Dieckhoff (Germany), Kees Camphuysen (The Netherlands), Eric Meek and Morag Wilson (Orkney), and Daniel M. Turner (northeast England). For providing other information, we thank: Niklas Aronsson (Sweden), Rolf Christensen and John Pedersen (Denmark), Aevar Petersen (Iceland), Jens-Kjeld Jensen and Bergur Olssen (Faroes). Alf Tore Mjøs, Bird Ringing Centre, Stavanger, Norway provided data on ring recoveries. In the UK, Roddy Mavor (JNCC) provided information on population trends and Jacquie Clark (BTO) on ringing recoveries [The BTO Ringing Scheme is funded by a partnership of the BTO, the JNCC (on behalf of: Council for Nature Conservation and the Countryside, the Countryside Council for Wales, Natural England and Scottish Natural Heritage), The National Parks and Wildlife Service (Ireland) and the ringers themselves]. Rob Barrett, Mike Harris and Andy Webb kindly commented on a draft of the manuscript.

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