State-dependent behaviour and energy expenditure: an experimental study of European robins on winter territories

1. We sought evidence for state-dependent behaviour and energy expenditure by manipulating the body-state of birds. We reviewed three hypotheses that could describe responses to a change in body-state: the Increased Effort; Reduced Effort; and Balanced Effort Hypotheses. 2. Territorial European robins Erithacus rubecula L. were captured at dusk during winter, and chilled or warmed overnight to manipulate body-state. 3. The robins were released onto their territories before dawn. Their subsequent behaviour was monitored by radio-tracking, and energy expenditure was measured using the doubly labelled water technique. 4. Robins with high overnight energy expenditures (= 'negative' body-state; chilled) were less likely to initiate vocal behaviour, and less likely to respond to the song or calls of conspecifics than those with low overnight expenditures (= 'positive' body-state; warmed). 5. Chilled birds moved around their territories at almost twice the rate of warmed birds, and had significantly higher daytime energy expenditures. 6. We concluded that behaviour, including vocalizations and activity levels, and energy expenditures of robins, were sensitive to manipulations of body-state. 7. These behaviours are interpreted as adaptive responses to restore condition, while minimizing the risk of mortality from predation and starvation. Our results were consistent with the Balanced Effort Hypothesis, entailing an increase in energy expenditure on foraging and a reduction in expenditure on non-maintenance activities. 8. We suggest that inter-individual differences in body-state may be a key cause of previously unexplained variation in daily energy expenditure.