Researchers curious about how small birds in the wild manage to survive cold winter nights have used miniature temperature transmitters and data loggers to solve the puzzle.
Prior to the study the researchers, from Curtin University and The University of Western Australia, suspected that like many small mammals the birds entered torpor – a period of lethargy – to lower their body temperatures and conserve energy to both survive the cold and cope with scarce food supplies during winter.
However co-author Dr Christine Cooper, of Curtin’s Department of Environment and Agriculture, said that after monitoring a population of White-browed babblers in a Western Australian forest for two consecutive winters, the researchers found that instead, the birds – which weigh on average about 46.5g – kept themselves warm by huddling in groups in tightly insulated nests.
“Insect-eating birds might be expected to use torpor in winter, like many mammals, because they’re small and lose heat easily. Their food is less abundant and they are inactive at night when it’s cold,” Dr Cooper said
“But even though babblers have characteristics that should make them perfect candidates for torpor, our research shows they rely instead on a combination of clever nest construction and communal lifestyles to maintain a body temperature of around 40.3C, even when it is below freezing outside the nest.”
The babblers were radio-tracked in the field at Dryandra Woodland south of Perth, and their body temperature measured remotely using temperature-sensitive transmitters.
The researchers also placed temperature loggers in the babblers’ nests to determine how cold they got. In the laboratory, they measured the birds’ overnight energy expenditure at different temperatures by analysing how much oxygen they used.
“We found they have low resting energy requirements and roost in large social groups in insulated, almost enclosed stick nests,” Dr Cooper said.
“This effectively halved their energy losses, allowing them to conserve 35-45 per cent of the energy consumed by individual birds. It played a big role in helping them to balance their daily energy budget and apparently negated any need for torpor.
“We now have a better understanding of the energy requirements of birds and their ability to keep their body temperature constant under different environmental conditions.
“It means we can better predict how temperature can impact on these birds.”
The study was led by Curtin University postgraduate student Tegan Douglas and supervised by Dr Cooper and Professor Phil Withers of The University of Western Australia. The resulting paper, Avian torpor or alternate thermoregulatory strategies for overwintering?, was published today in the Journal of Experimental Biology.