Great Tit Fat Reserves: Effects of Changing and Unpredictable Feeding Day Length

In two experiments, we tested the hypothesis that Great Tits (Parus major) adjust their fat reserves in response to feeding conditions. Experiment 1 tested the effect of a change in mean time at which food became available in the morning. Experiment 2 tested the effect of unpredictability in the onset of feeding time. In experiment 1, Great Tits increased their morning mass after the start of feeding was delayed by 3 h and decreased their mass when original conditions were restored. The response to shortened feeding day was more rapid than the response to lengthened day. On shortened feeding days, birds gained mass while eating less, suggesting that they reduced their expenditures. In experiment 2, birds gained more mass per day during the unpredictable than during the constant treatment. The performance of individuals was correlated across the two experiments, suggesting that the birds reacted to the mean and unpredictability in feeding conditions using the same mechanism. This mechanism might involve a more rapid response to worsening than to improving conditions.