Behavioural control of daily fattening in great tits (Parus major)

Many species of temperate-zone passerines show a pronounced daily cycle in body mass. Energy reserves are built up during the day and consumed the following night. The size of reserves is often viewed as a compromise between the risk of starvation and the cost of carrying an excessive fat load. This trade-off calls for state-dependent foraging behaviour, where current reserves and time of day: are two crucial factors. The foraging strategy of the birds may then be reflected by the pattern of daily mass gain rate. We temporarily increased energy expenditure in captive great tits (Parus major) by experimentally lowering the overnight temperature. The birds' response to the treatment was to rapidly compensate for reduced morning reserves. Such an increased rate of mass gain suggests state-dependent foraging, and that some feeding opportunities are normally rejected. The rate of return to the normal pattern of fat accumulation suggests that in these birds, foraging is not constrained by physiological limitations.