Torpor in a passerine bird expands the scope of avian heterothermy

Torpor is a controlled physiological state of reduced metabolism and typically body temperature (T_b) used by some endothermic animals to reduce their resting energy expenditure^1,2 and increase survival during unfavorable environmental conditions.^3,4,5,6 Passerine birds, the largest avian order, have long been considered limited to shallow reductions of T_b (min. T_b usually >30°C; undocumented <23°C) during part of their rest phase,^7,8 with deeper and longer torpor restricted to a few non-passerine bird groups.⁹ Here, we used temperature telemetry to record the skin temperature (T_s; as a close proxy for core T_b) of free-living white-backed swallows (Cheramoeca leucosterna; 15 g) during winter in inland Australia. This species is an aerial foraging passerine bird that roosts in burrows. On most nights, we measured only shallow reductions in T_s; however, during and after rainfall events, tagged birds remained inactive in their burrows during the daytime and employed bouts of torpor that were relatively deep (min. T_s: 18.8°C) and long (max.: 17.5 h). The depth of torpor and extension of torpor beyond the nocturnal rest phase expand the known capabilities for heterothermy within passerine birds. For aerial foraging swallows, torpor is presumably an important adaptation for reducing starvation risk during weather events that reduce prey availability. Our findings have implications for understanding avian thermoregulatory strategies, the evolution of torpor, and how endothermic animals cope with extreme weather events. They also highlight the power of biologging to reveal previously unknown physiological capabilities and provide insight into the adaptive importance of animal responses to natural environmental conditions.