Great Tit Fat Reserves under Unpredictable Temperatures

We tested the effects of unpredictable temperatures on fat reserves in Great Tits Parus major. During one treatment, the temperature was constant at 8.5°C. In the other, temperatures fluctuated between 1.5 and 15.5° on a 24-h basis, with changes occurring just after lights-out. Residual evening weights were higher during the period of unpredictable temperatures. At the end of the period with unpredictable temperatures, more weight was gained on cold than on warm days. During the unpredictable temperature treatment, birds defecated less while eating the same amount. Nightly weight loss depended upon evening weight level, but not upon overnight temperature. Our results suggest that Great Tits use daily temperatures to predict conditions for the following night and that they regulate overnight expenditures to match reserve levels.