| Summary: | A Knot Calidris canutus, kept with four conspecifics on an en-closed artificial outdoor tidal flat in The Netherlands, refused to feed on the available bivalve prey for a period of 18 days and thereby decreased in mass from 209 g to 107 g, at which point the bird resumed feeding on the then freely available food pellets. The bird had been captured in South Africa a few months before the experiment, and the start of its voluntary fast may have mimicked the natural mass decrease during northward migration. After 14 days of fasting, when the bird had reached a mass of 129 g, it started making numerous flights through the cage as if wanting to escape. Since this is the mass level at which the fat stores of Knots approach depletion and since there is evidence that the focal bird was beginning to catabolize its pro-tein reserves, the restlessness of the experimental Knot strongly parallels the behaviour and physiology of Aptenodytes-penguins (and, incidentally, the behaviour of Oystercatchers Haematopus ostralegus). When engaged in en-forced or voluntary fasts, all these species also become restless when fat levels decrease to 8-9% of body mass and (in penguins) when an increase in the catabolism of protein begins. This hints at a basic avian physiological and behavioural mechanism regulating the initiation of migratory move-ments in the face of endogenous nutrient depletion and food scarcity. Eluci-dating these mechanisms will shed light on the adaptive value of the behav-iour patterns observed in the wild.
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