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1. Knowledge of the functional response of predators to prey densities conditions our understanding of food webs. Such links are still poorly understood within the higher trophic levels of marine ecosystems.2. We present the first field study recording the foraging effort and foraging yield of a seabird (the Great Cormorant,Phalacrocorax carbo) as well as the abundance and quality of prey within its foraging area.3. We confirm that Great Cormorants foraging off West-Greenland show the highest foraging performance recorded for a marine predator (between 17 and 41 g fish caught per minute underwater). Former work suggests that such high foraging yield should be based upon the exploitation of extremely profitable prey patches.4. Contrary to this hypothesis, average prey abundances estimated within the foraging areas of the cormorants were low (0·03–0·09 prey m−2, depending on methods), as was the average calorific value of the prey items (4·2 kJ g−1).5. Our study suggests that Great Cormorants remain highly successful predators even when exploiting modest prey resources. These findings have implications for our understanding of predator–prey relationships, and for the management of Great Cormorant populations.Functional Ecology(2004)18, 793–801 [ABSTRACT FROM AUTHOR]