Among birds, pronounced heterothermy tends to be used by small species belonging to phylogenetically older taxa and inhabiting environments with unpredictable and/or seasonally scarce food resources. Previous evidence suggests that a small arid-zone raptor, the pygmy falcon (Polihierax semitorquatus), enters torpor and decreases its body temperature (Tb) to below 31 °C on winter nights in the Kalahari Desert, where sub-zero night-time temperatures occur regularly. To confrm that this species has the capacity for heterothermy and to characterise its thermoregulatory patterns under natural conditions, we implanted temperature-sensitive radio transmitters into free-ranging falcons. Based on data from seven individuals, we found no evidence for the substantial reductions in Tb previously reported. The mean minimum rest-phase Tb during the study period was 37.89±0.67 °C, we observed no instances of Tb<36 °C, and air temperature did not signifcantly infuence patterns of thermoregulation. Our results highlight intraspecifc variability in heterothermy among birds and reveal that, contrary to previous suggestions, even in mid-winter pygmy falcons may not reduce Tb below normothermic levels.
- Body temperature