Heterothermy in an Australian passerine, the Dusky Woodswallow ('Artamus cyanopterus')

Information regarding passerine heterothermy and torpor is scant, although many species are small and must cope with a fluctuating food supply and presumably would benefit from energy savings afforded by torpor. We studied whether insectivorous Dusky Woodswallows (Artamus cyanopterus; ~35 g) enter spontaneous torpor (food ad libitum) when held outdoors as a pair in autumn/winter. Woodswallows displayed pronounced and regular daily fluctuations in body temperature (Tb) over the entire study period. The mean Tb ranged from ~39°C to 40°C (photophase, day time) and ~33°C to 36°C (scotophase, night time). However, on 88% of bird nights, nocturnal Tb minima fell to < 35°C. The lowest Tb observed in air was 29.2°C. However, when a bird fell into water its Tb dropped further to ~22°C; this Tb was regulated for several hours and the bird survived. Our observations suggest that heterothermy is a normal part of the daily thermal regime for woodswallows to minimise energy expenditure. Spontaneous nocturnal torpor in captive woodswallows suggests that torpor in the wild may be more pronounced than recorded here because free-living birds are likely challenged by both low food availability and adverse weather.