On days when it just feels too hot, it's good to find a pool to lounge by. But as the mercury climbs, most creatures don't have that luxury. Southern pied babblers (Turdoides bicolor) have to keep going, foraging and caring for their young. But as temperatures rise further, the birds from southern Africa begin to struggle, lose weight and, at temperatures above 38°C, are even unable to breed. Are these overheated birds experiencing the kind of stress usually associated with physical threats, such as an attack? ‘Stress responses mediated by hormones are a vital component of an animal's reactions to hazards’, says Lesedi Moagi from Tshwane University of Technology, South Africa, adding that the brief hormone bursts triggered when an animal feels threatened release essential energy to help them to evade peril. The question was, do extreme high temperatures trigger the same physiological response as physical risks in wild southern pied babblers?
Heading out to the Kuruman River Reserve in the Kalahari Desert, Moagi and Amanda Bourne from the University of Cape Town, South Africa, collected almost 900 fresh droppings from more than 70 babblers over a 4 month period in summer temperatures ranging from 28 to 41°C. ‘The babblers are colour-ringed for individual identification and thoroughly habituated to the presence of researchers, allowing observation of the birds from distances of just a few metres’, says Amanda Ridley of the University of Western Australia, who established the study population in the early 2000s. Back in the lab, Moagi measured the level of stress hormone breakdown products in the birds’ faeces and discovered that the birds were really beginning to suffer at temperatures over 38°C; their stress hormone levels had climbed dramatically. However, when the team checked the poo for signs of distress the following day, the babblers seemed to have got over the preceding heatwave.
But Susie Cunningham from the University of Cape Town, South Africa, warns that the risk from high temperatures can set in even before air temperatures reach a blistering 38°C. ‘Babblers typically lose ∼4% of body mass overnight, with reduced foraging success on hot days above 35.5°C, creating conditions under which they may fail to regain those loses, resulting in progressive loss of condition during periods of sustained hot weather’, she says.
So, super-hot days are more than just a bit uncomfortable for desert-dwelling babblers, they are positively stressful and the team suggests that scientists keep track of bird stress hormone levels to spot when species are really beginning to struggle. Andrew McKechnie from the University of Pretoria, South Africa, says, ‘Quantifying heat stress could prove a useful tool for managers and conservationists seeking to evaluate the effect of management interventions such as supplementary feeding or the provision of artificial water or shade, particularly for threatened range-restricted species in hot climates’.