The combination of short days and long cold nights in winter, in temperate regions, presents a major challenge for small diurnal birds. Small birds regularly employ heterothermy and enter rest-phase hypothermia during winter nights to conserve energy. However, we know little about how environmental conditions, such as food availability, shape these strategies.

Using an experimental set-up, we manipulated food availability in winter to free-living great tits Parus major. A “predictable” and constant food supply was provided to birds in one area of a forest, while birds in another area did not have access to a reliable supplementary food source. We found that predictability of food affected the extent of nocturnal hypothermia, but the response differed between sexes. Whereas male nocturnal body temperature was similar regardless of food availability, females exposed to a naturally “unpredictable” food supply entered deeper hypothermia at night, compared with females that had access to predictable food and compared with males in both treatment groups. We suggest that this response likely is a consequence of dominance, and the subdominant females cannot maintain sufficient energy intake, resulting in a higher demand for energy conservation at night.

Body temperature measurements of wild great tits in relation to predictability of food supply.