The ecology of British Cormorants Phalacrocorax carbo
outside the breeding season, as shown by ring recoveries

Chris V. Wernham & Will J. Peach

BTO, The Nunnery, Thetford, Norfolk, IP24 2PU, UK; Email: chris.wernham@bto.org

There is believed to have been a large increase in the numbers of Cormorants wintering in Britain in the last 20-30 years. This has increased the perception that Cormorants are affecting fish stocks, particularly inland in England. We use more than 8000 recoveries from the 55,000 Cormorants ringed in Britain and Ireland since 1909, and those of birds ringed in continental Europe and recovered in Britain, to investigate movements outside the breeding season and annual survival rates. Between 1965 and the early 1980s, the proportion of Cormorants recovered inland in Britain increased significantly, even after controlling for differences in hunting pressure between inland and coastal areas. Cormorants from various natal regions differ in their probabilitiy of being recovered inland. Nevertheless, the inland-wintering birds do not come from single natal regions, and the origins of inland winterers are particularly mixed in areas without adjacent coasts supporting large Cormorant colonies. Recoveries within Britain Cormorants ringed in continental Europe suggest that as few as 2% of the Cormorants wintering in Britain are of direct continental origin. Stabilisation of the proportion wintering inland and falling survival rates suggest that the conflict with inland fisheries is unlikely to worsen. Gaps in current knowledge that could be filled by analysis of existing ring-recovery data and by new data collection are discussed.