SMALL birds increase their fat reserves in winter as insurance against reduced or unpredictable food supplies(1) : fat is accumulated daily from feeding and utilized overnight(2). Field observations indicate that birds often maintain smaller reserves than expected(2), which implies that there is a cost of being fat(3). One such cost could be that an increased fat load reduces manoeuvrability, thus increasing the risk of predation(3,4), Here we demonstrate a link between fat reserves and predation risk by describing changes in body mass (roughly equivalent to fat reserves) that have occurred in British populations of the great tit, Parus major since 1950, a period when the numbers of its principal predator, the sparrowhawk Accipiter nisus, changed markedly, Furthermore, these changes resulted from individual tits adjusting their mass, rather than from the selection of heavier great tits by hawks.