Nocturnal body temperature in wintering blue tits is affected by roost-site temperature and body reserves

Birds commonly use rest-phase hypothermia, a controlled reduction of body temperature (T(b)), to conserve energy during times of high metabolic demands. We assessed the flexibility of this heterothermic strategy by increasing roost-site temperature and recording the subsequent T(b) changes in wintering blue tits (Cyanistes caeruleus L.), assuming that blue tits would respond to treatment by increasing T(b). We found that birds increased T(b) when roost-site temperature was increased, but only at low ambient temperatures. Moreover, birds with larger fat reserves regulated T(b) at higher levels than birds carrying less fat. This result implies that a roosting blue tit maintains its T(b) at the highest affordable level, as determined by the interacting effect of ecophysiological costs associated with rest-phase hypothermia and energy reserves, in order to minimize potential fitness costs associated with a low T(b).