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).