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 (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.