Mass mortality of adult Razorbills Alca torda in the Skagerrak and North Sea area, autumn 2007

Abstract

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.