Large seabirds such as northern gannets Morus bassanus have very flexible time–activity budgets; this means that changes in variables such as foraging-trip duration could provide a rapid indicator of changes in food supply, an indicator that could not be obtained from smaller species. Moreover, larger birds often have longer foraging ranges, giving them the potential to integrate information about changes in food availability over large areas of ocean. There is insufficient information on temporal variation in fish stocks exploited by far-ranging species to determine how the birds' foraging ecology varies with prey abundance, but comparing colonies of different size potentially presents an opportunity to examine empirically how foraging ecology varies in relation to prey availability. For gannets in Britain and Ireland, there were major differences between colonies in foraging and food-provisioning behaviour, but the relationship between trip duration and foraging range was remarkably constant. Moreover, there was a strong relationship between trip duration and the square root of colony size, which was very similar within colonies between years and between colonies within a single year. This relationship could provide a powerful tool for gauging the importance of changes in trip duration in terms of changes in per-capita prey availability. Over 4 years, annual variation in diet at one colony to some extent reflected variation in trip durations and foraging locations, although one year was anomalous and a combination of trip duration and diet provided a much more complete picture than either did on their own.
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