Predation risk and the cost of being fat

SMALL birds increase their fat reserves in winter as insurance against reduced or unpredictable food supplies1: fat is accumulated daily from feeding and utilized overnight2. Field observations indicate that birds often maintain smaller reserves than expected2, which implies that there is a cost of being fat3. One such cost could be that an increased fat load reduces manoeuvrability, thus increasing the risk of predation3,4. 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 spar-rowhawk 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.