Long-term study of mortality in thecommon guillemot in the Baltic Sea

This study is based on a uniquely large and long-term data set ofringed common guillemots (Uria aalge) in the Baltic Sea. We analysedrecoveries of 42,824 common guillemots ringed in Sweden from1912-1998. Most of the birds were juveniles ringed on the island ofStora Karlso in the southern Baltic proper. Of the ringed birds, 6 %were recovered (i.e. found dead or controlled alive).Our data clearly indicated that almost all common guillemots thatbreed in the Baltic Sea area stay in the region all year round. Outsidethe breeding season, we found significant differences in the distributionof recoveries of birds up to two years old compared to olderbirds, which indicates that the younger birds tend to winter furtheraway. In winter (September-February) birds of all age classes wererecovered mainly in the central and southern parts of the Baltic proper:off the Swedish islands of Gotland and Oland (including thefishing banks south of Gotland), the Polish Gulf of Danzig, off theDanish island of Bornholm, in the Swedish Hanobukten Bay, offthe German island of Rugen and in the Pomarian Bay, and off theDanish islands.The recoveries reported during the summer ( June and July) indicatedthat as the birds grew older, their distribution became less spreadout:recoveries (including recaptures) of birds that were five yearsand older were concentrated to known breeding localities in summer.This supports previous suggestions that common guillemotsdo not start breeding until about five years of age.

Olof Olsson, Torbjorn Nilsson, Thord Fransson, Swedish environmental protection agency