Abstract
Metcalfe & Ure (1995) reported strong effects of natural body mass variation on time to fly to cover by zebra finches. They observed that escape time was a roughly linear function of body mass whereas models have often assumed accelerating relations between fat reserves and predation risk. To translate escape time into predation risk, one needs to know the distribution of distances at which approaching predators are detected. This depends on the vigilance behaviour of the prey. Here I show that predation risk per attack will increase exponentially with time to reach protective cover for a commonly observed pattern of scanning by birds. Besides influencing fattening decisions, this rapidly increasing relation may also explain the observed reluctance of many foragers to venture more than a few metres from protective cover. The data presented by Metcalfe & Ure are perfectly compatible with recent theory and perhaps even more important than stated by the authors.
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