Birds tend to run hot. With their higher metabolic rates and feather insulation, their body temperatures are routinely up to 41°C. But with estimates suggesting that 2015 could be the hottest year on record and with extreme thermal events on the increase, no one is sure how our feathered friends will fare in the future. ‘These [extreme] events produce direct mortality in birds, megabats and other wildlife on several continents’, says Eric Smith from the University of New Mexico, USA. Concerned about the impact of environmental change on desert bird populations, Blair Wolf and a team of international collaborators from Australia, South Africa and the USA, have started investigating the effects of extreme heat on birds and how environmental change will affect which habitats they occupy. Knowing that birds lose heat from their bodies by evaporating water, either while panting or directly across the skin, Wolf, Smith and Alexander Gerson decided to investigate the thermal tolerances and water losses of three desert species.
Relocating the lab to the deadly Sonoran Desert – where temperatures can reach a scorching 49°C – Smith, Gerson, Wolf and undergraduate Jacqueline O'Neill lured Gambel's quails and two species of dove (mourning doves and white-winged doves) into traps laced with grain. Then, they gently placed individual birds in a respirometry chamber housed in a modified ice chest where they could precisely regulate the air temperature while recording the birds’ water losses and metabolic rates as they gradually increased the temperature from a comfortable 30°C to temperatures exceeding 60°C. ‘Some birds showed varying levels of activity from resting quietly to intermittent flapping escape attempts’, Smith recalls. However, he admits that he was astonished when the first mourning dove that they examined at the blistering temperature of 60°C appeared perfectly content; ‘it was an incredible sight’, he says.
Analysing the birds' metabolic rate traces relative to the ice chest temperature, the team realised that the doves were much better prepared for a hotter future than the quails. The doves’ metabolic rate increased dramatically at temperatures above 45.9–46.5°C as they dealt with the physical burden of the heat, while the quails’ metabolic rate began rising significantly at the lower temperature of 41.1°C. And when they investigated the birds’ weight-adjusted water loss rates, they could see that the doves lost 30–45% more water than the quails to keep cool. ‘Higher rates of evaporation and higher upper critical temperatures made the doves exceptionally heat tolerant, allowing them to maintain body temperatures at least 14°C below air temperatures as high as 60°C’, says Smith.
Explaining that most birds, quail included, actively pant to lose heat, Smith says, ‘Panting is metabolically costly and produces its own heat’. However, doves lose water by evaporation across the skin, and Smith suspects that this could tip the doves a metabolic advantage. ‘Evaporating water from the skin appears to have negligible metabolic costs’, he explains, adding that the passively evaporating doves can dissipate heat loads that are greater than three times their own thermal output, whereas panting quail can only deal with heat loads double theirs.
However, he adds that doves do pay a price for their ability to withstand the heat. ‘Doves…support a high-water-use “lifestyle” that could reduce their abundances if water were unavailable’, he explains, adding, ‘It is critical that we understand the thermoregulatory and water balance challenges that birds face in a rapidly warming world…it is a prerequisite for understanding avian distributions now and in a warmer future’. Who knows, maybe doves will inherit the Earth after all.
