Torpor use in response to predation risk in a small, free-living bird: rest-phase energy savings moderate the costs of active-phase behavioural decisions

Animal decisions trade-off the mortality risks of starvation and predation, and anti-predator behaviours generally incur a cost of reduced energy intake. Torpor is a widespread physiological mechanism for reducing starvation risk among small mammals and birds. Here, we present a field-based experimental test of the hypothesis that energy savings from torpor use can also reduce predation risk by moderating the energy cost of anti-predator behavioural responses in a small bird during winter. We manipulated perceived predation risk in wild populations of the superb fairy-wren (Malurus cyaneus) by playback of conspecific alarm calls during the daytime active-phase and tested for effects on body temperature measured continuously by telemetry during the nocturnal rest-phase. We found that alarm call playback was associated with subsequent rest-phase torpor bouts that were significantly deeper (minimum skin temperature: 28.7 ± 1.7 °C vs. 30.0 ± 1.5 °C) and longer (duration in torpor: 6.0 ± 2.7 h vs. 3.8 ± 2.3 h) compared to control periods. By demonstrating the connection between resting energy expenditure and energy costs of behavioural decisions during activity, our study has implications for understanding both the ecological functions of torpor and the mortality risk consequences of behavioural responses by small birds to environmental challenges.