We studied the British and Irish seabird community in the 20th century, a period of great anthropogenic impact. We determined the average body mass of community members and analysed population sizes, phylogenetic and spatial structures. We also quantified the total predation exerted by seabirds around Britain and Ireland and the spatial distribution of this predation in the North Sea. To achieve these aims we used (1) presence or absence of the seabird species in the different counties of Britain and Ireland between 1875 to 1900 and 1968 to 1972, (2) seabird breeding censuses of Britain and Ireland from 1969 to 1970, 1985 to 1988 and 1998 to 2002, (3) at-sea abundance and distribution surveys of seabirds in the North Sea from 1980 to 1985 and 1990 to 1995, and (4) a bioenergetics model to estimate energy expenditures for 40 seabird species. Our analyses suggest a marked expansion in the breeding range of seabirds in Britain and Ireland between 1875 and 1972. Total seabird numbers also increased at an average rate of 1% per annum between 1969 and 2002, with a related increase of 115% in predicted total seabird predation. Only terns Sternidae declined during this second period. Some characteristics of the community (geographical and phylogenetic structure, body mass) showed minor and non-significant variability between 1969 and 2002. Finally, seabird predation in the North Sea showed a slight north-eastward shift between 1980 to 1985 and 1990 to 1995. Overall, our study indicates that the seabird community of Britain and Ireland has prospered during the 20th century. These results contrast with extensive breeding failures recorded during the first years of the 21st century, which indicate that certain species within the community are now being critically disturbed
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