Body mass regulation and the daily singing routines of European robins

Stochastic dynamic programming (SDP) models of daily singing and foraging routines in birds relate an individual's fat reserves to the relative costs and benefits of singing and foraging at different times of day. Two central predictions of such models are that: (1) overnight loss of fat reserves is higher on colder nights, and (2) birds sing more at dawn when their fat reserves are high. We tested these predictions in free-living European robins, Erithacus rubecula, by examining the relationships between ambient temperature, body mass change (an index of changes in fat reserves) and song rate. In support of prediction (1), overnight mass loss was positively associated with overnight temperature. However, robins also put on more mass at dusk if the night ahead was going to be cold, which tended to buffer the effects of overnight temperature on dawn body mass. In support of prediction (2), robins sang more at dawn on days when their dawn body mass was high, although this association was detectable only after controlling for foraging intensity during the dawn chorus. We analyse differences between the results of this and related studies and discuss the implications of these results for existing SDP models of daily singing routines. (C) 2002 The Association for the Study of Animal Behaviour.