Many seabirds in the North Sea feed on lesser sandeels Ammodytes marinus during the breed- ing season. Unprecedented breeding failures were recorded at many seabird colonies on the east coast of Britain in 2004. We used demographic, dietary and behavioural data from a long-term study of a colony of common guillemots Uria aalge, the most abundant seabird species in the North Sea, to set the 2004 season in context. Birds at this colony showed greatly reduced breeding success and those chicks that did survive left the colony in very poor condition. The main prey item fed to chicks in 2004 was sprat Sprattus sprattus rather than sandeels, and parents increased the amount of time spent foraging, frequently leaving chicks unat- tended in order to maintain a normal feeding rate. The calculated daily food intake of chicks derived from these values did not differ markedly from previous years and therefore the magnitude of the impact on chick growth and breeding success appeared dispro- portionately large. However, nutrient analyses of fish collected from birds in 2004 revealed them to be of sig- nificantly lower energy value than expected. Poor food quality therefore appeared to be the proximate cause of seabird breeding failure in 2004 giving support to the ‘junk-food’ hypothesis. Single-prey loaders such as guillemots will be particularly sensitive to reductions in the energy value of food items. The reasons for the poor fish condition in this part of the North Sea are currently unknown, but the results provide further evidence of major changes in the North Sea food web.
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