Nature 377, 621 - 623 (19 October 2002); doi:10.1038/377621a0

Predation risk and the cost of being fat

Andrew G Gosler^*, Jeremy J. D. Greenwood^† & Christopher Perrins^*

^*Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford 0X1 3PS, UK
^†British Trust for Ornithology, National Centre for Ornithology, Nunnery Place, Thetford, Norfolk IP24 2PU, UK

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 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 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.

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