Daily routines of body mass gain in birds: 2. An experiment with reduced food availability

Theoretical models predict that small birds should adjust daily patterns of body mass gain in response to environmental and internal factors. In a companion paper, we described a model on daily fattening that allows the analysis of precise changes in the shape of hourly patterns of body mass. In this study, we tested one of the main predictions of the model: the inflexion point of the body mass trajectory should be delayed in response to a decrease in food availability, increasing body mass as soon as possible when food is scarce. This effect might be stronger in subordinate than in dominant birds. We tested both predictions with four pairs of coal tits, Periparus ater, kept in cages where food was delivered at high and low rates. Daytime increase in body mass was 1 g in both treatments. As predicted by the model, the tits increased body mass as soon as possible when food was delivered at a low rate, and the inflexion point of the body mass trajectory was 16.7% delayed compared to the high food delivery rate. However, dominance rank had no significant effect on the shape of daily body mass increase. To our knowledge, this is the first precise estimate of the change in the shape of body mass trajectories. Our findings have important implications for the analysis of daily patterns of body mass and for the design of studies of body mass in small birds.