Winter thermoregulation in free-ranging pygmy falcons in the Kalahari Desert

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 (T_b) to below 31 °C on winter nights in the Kalahari Desert, where sub-zero night-time temperatures occur regularly. To confirm 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 T_b previously reported. The mean minimum rest-phase T_b during the study period was 37.89 ± 0.67 °C, we observed no instances of T_b < 36 °C, and air temperature did not significantly influence patterns of thermoregulation. Our results highlight intraspecific variability in heterothermy among birds and reveal that, contrary to previous suggestions, even in mid-winter pygmy falcons may not reduce T_b below normothermic levels.