Following several recent major oil spills affecting British & Irish breeding Guillemots Uria aalge, I carried out analyses of the origin of ringed birds recovered following these incidents to describe the importance of different wintering areas for this species. The results show significant differences in the age structure and natal origin of birds affected in the different oil spills, indicating the existence of different wintering areas for different breeding populations of Guillemots. Birds wintering in the south western approaches to the English Channel and south into the Bay of Biscay (affected by the Prestige spill) tended to be immature birds from colonies in the west of Britain and Ireland, whereas birds wintering in the English Channel and North Sea (affected by the Tricolor spill) tended to be adults from colonies in eastern Britain. In general, immatures appear to winter further from their natal colony than adults. By understanding the dispersal patterns and winter distribution of such birds, we can assess the likely impacts on bird populations of pollution incidents.