Jd. Godfrey et Dm. Bryant, State-dependent behaviour and energy expenditure: an experimental study ofEuropean robins on winter territories, J ANIM ECOL, 69(2), 2000, pp. 301-313
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; R
educed 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 subse
quent 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 bod
y-state.
7. These behaviours are interpreted as adaptive responses to restore condit
ion, 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 expenditur
e 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.