Plastic daily fattening routines in willow tits

Summary1 Each day small birds must build up sufficient energy reserves, largely fat deposits, for the following night and for the periods when foraging is not possible. The earlier the birds acquire the reserves and the more they carry them, the higher is the insurance value of the reserves. Still, small birds do not forage at maximal capacity during the day and, even at dusk, body reserves are usually below the physiological upper limit. A popular explanation for this is that birds’ fatness levels reflect an optimal compromise between starvation risk and risks associated with reserve carrying and acquisition. 2 Theoretical works suggest that uncertain or variable feeding conditions should promote high reserve levels, and that in the daily perspective, feeding should peak in the morning hours. In more favourable conditions, reserve levels should be smaller and the acquisition peak should be delayed towards the end of the day. These predictions were tested by experimentally manipulating winter feeding conditions and monitoring subsequent daily body mass increase in northern population of the willow tit Parus montanus (Conrad), a small‐sized forest passerine. 3 The birds using feeders appeared heavier than the control birds throughout the day. This contradicts the prevailing theory, but is understandable since the feeding procedure probably not only affected food amount but may also have decreased acquisition costs in terms of predation. 4 Daily mass gain and the gain rates were fairly similar in the food‐manipulated and the control birds. However, the mass gain schedules differed between the treatment groups: the food‐supplemented birds gained mass at a roughly constant rate while among the control birds the gain rate was higher in the morning than in the afternoon. Probably to avoid associated costs, birds abandon early build‐up of reserves if foraging conditions become favourable enough.