Some species are notorious huddlers, from bats that nestle together for warmth to enigmatic emperor penguins braced against the harsh Antarctic winter. Yet others, such as chestnut-crowned babblers from southeastern Australia, continue huddling even when the nights are mild. Mark Chappell from the University of California, Riverside, USA, and colleagues William Buttemer at the University of Wollongong, Australia, and Andrew Russell from the University of Exeter, UK, explain that little was known about the interplay between factors such as environmental temperature and breeding habits that drive roosting, so the trio began investigating how the chestnut-crowned babblers’ sociable lifestyle affects their metabolism.
After carefully capturing birds at the University of New South Wales Research Station at Fowlers Gap, the team measured the metabolic rates of individual unsheltered birds and groups of birds (up to 9) sheltered in nests. Conducting the trials overnight, the trio initially set the temperature at a chilly 5°C (typical of the early breeding season) as they recorded the birds’ oxygen consumption, before raising the temperature to 14°C, and then completing the measurements several hours later at the warmer temperatures (28°C) that the birds encounter late in the breeding season.
Not surprisingly, the individual birds had to work harder to stay warm in the chilly conditions, consuming 112% more energy at 5°C than they did at 28°C. And when the scientists moved the solitary animals into nests, their metabolic rates fell by between 11% and 15%, depending on the temperature. However, the birds’ metabolic rates plummeted when the number of nest mates increased, with large groups of birds in the coldest nests consuming 50–60% less energy than the solitary birds at the same temperature.
The team also noticed that the metabolic rates of the individual birds were extremely variable and their resting metabolic rates were 15% higher than those of birds huddling in a nest at temperatures where it should not have been necessary for them to resort to shivering for warmth, which may suggest that nesting alone is stressful. Chappell and colleagues also suggest that this could account for the relatively high mortality rates suffered by chestnut-crowned babbler mothers as the females roost alone while rearing their young, and the additional expense incurred when nesting in the chilly early breeding season could place them at significant risk.