Australia is a continent blessed with animals that operate at a slow pace; from koalas that spend up to 20 h a day sleeping, to antechinus marsupials that drop their body temperature and resort to torpor in the aftermath of an inferno, many of the continent's smaller animals have opted for a sedate lifestyle to eke out meagre resources. But how do birds cope when nutritious insect populations dwindle as the temperature falls? Do they resort to torpor to conserve energy or have they evolved alternative survival strategies that help pull them through lean periods? Intrigued by the possibility that birds may allow their body temperature to drop to conserve energy (i.e. become torpid) during winter, Tegan Douglas and Christine Cooper from Curtin University, Australia, and Phil Withers from the University of Western Australia fitted white-browed babblers (Pomatostomus superciliosus) with minute temperature loggers to record their body temperature over the course of consecutive winters.
However, after two cold seasons it was clear that the small birds did not resort to lowering their body temperature to conserve energy, maintaining a toasty body temperature around 40.3°C, even when the temperature outside the nest fell below freezing. So, how did the birds maintain their body warmth under the difficult winter conditions?
Measuring the metabolic rate of white-browed babblers in the lab, the trio discovered that it was 64% lower than expected for similarly sized birds (46.5 g) and they also lost water at a slower rate, both characteristics that should make the birds ideal torpor candidates. However, Cooper and her colleagues suspect that the secret behind the birds’ high body temperature is their communal lifestyle as they huddle together, which allows them to conserve 35–45% of the energy consumed by individuals. And when the team measured the impact of the birds’ almost enclosed stick nests on their metabolic budget, they realised that the structures halved the animals’ energy losses.
‘These substantial energy savings, together with their [the birds’] intrinsically low BMR would play an important role in babblers balancing their daily energy budget and presumably negate any requirement for torpor in their energetically challenging environment’, says Cooper. The trio concludes that many of Australia's small birds may resort to similar strategies to avoid using torpor to keep their energy budgets balanced when food is scarce and conditions are harsh.
