An experimental test of state-behaviour feedbacks: gizzard mass and foraging behaviour in red knots

1. Animals frequently exhibit consistent among-individual differences in behavioural and physiological traits that are inherently flexible. Why should individuals differ consistently in their expression of labile traits? Recently, positive feedbacks between state and behaviour have been proposed as a parsimonious explanation for the maintenance of consistent among-individual differences in both state and behaviour. If state affects behaviour, and behaviour reciprocally affects state, then even chance differences in either state or behaviour that arise among-individuals could be maintained over extended periods of time.
2. We tested for positive feedbacks experimentally using wild-caught red knots (Calidris canutus islandica). In the wild, knots exhibit consistent among-individual differences in digestive physiology (gizzard mass) and foraging behaviour (diet), two inherently labile traits.
3. Experimentally manipulated diet quality had a large effect on gizzard mass. Experimentally ma...