Ways to beat heat have hidden costs for birds

WASHINGTON — In the short-term, ways to beat the heat are cool. But for desert birds, even simple panting or flying into the shade have some sneaky long-term costs.

When male southern yellow-billed hornbills pant, they’re less able to snap up food, Susan Cunningham reported August 18 at the North American Ornithological Conference. The hornbills are the third bird species that Cunningham, of the University of Cape Town in South Africa, and various colleagues have shown face hidden costs of trying not to overheat.

Birds certainly have ways to ease the immediate dangers of heat. But determining the full consequences of all those small accommodations becomes more urgent as the climate changes.

Yellow-billed hornbills (Tockus leucomelas) could be especially vulnerable to hidden costs of heat because males become the sole provisioners of their families during breeding season. A female walls herself and her eggs into a cavity (or a research nest box), leaving open a hole big enough only for her mate to poke food through. In southern Africa’s Kalahari region, a female may stay walled in the cavity for a month or more, leaving the male to scour hot, dry land for her food, his own and eventually, the chicks’.

During bouts of panting, males caught less food than they did in minutes before or after while not panting, Cunningham’s student Tanja van de Ven has found. A specially rigged perch on nest boxes registered a male’s weight every time he landed on it. When the temperature rose above 36.5° Celsius, males typically failed to maintain weight, raising concerns about their ability to care not only for themselves, but for dependents, too.

Cunningham had already seen costs of chronic panting in another Kalahari species, the southern pied babbler (Turdoides bicolor). A population of these social birds had become blasé about nearby scientists taking notes thanks to patient effort by Amanda Ridley of the University of Western Australia in Crawley. The birds even hopped onto a portable scale, allowing weight monitoring in the field. Ridley, Cunningham and other babbler chroniclers found that on hot days, the birds persevered in foraging but caught less for their effort. As temperatures rose above 35.5° C, the scales showed that birds were struggling to maintain body weight. Typically they lost more weight overnight than they could make up during a day, the team reported in 2012

Even a cooling strategy as simple as taking shelter in the shade can cut into a bird’s ability to collect resources in arid lands. Southern fiscals (Lanius collaris), chunky predators with fierce bills, prefer high, sunny perches from which to scan for the rodents and insects they attack in lightning-bolt dives. As the Kalahari’s temperatures rose, these birds spent more and more time on shady perches, usually lower to the ground. The birds didn’t catch as much from such spots, the researchers found, and growth slowed among younger chicks when parents had to hunt from substandard posts. Each day with a temperature above 35° C kept chicks in the nests a half day longer. That’s perilous, Cunningham and colleagues argued in a 2013 report on fiscals. The risk that predators will scavenge a nest, or it will fail in some other way, increases 4 percent for every day the chicks remain in it.

Uncovering the downsides of such simple behaviors “is pretty cool,” said Blair Wolf of the University of New Mexico in Albuquerque. Birds can’t sweat, he pointed out, and the many species that pant as a cooldown technique have to compensate for the water lost in the process. Birds let their body temperatures rise to heights that would cook a human, and Wolf’s work has shown that this tolerance lessens water loss. Whether there are some hidden costs to this heat-fighting measure, as there are to panting and shade-seeking, remains to be seen.

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Editor’s note: This story was updated August 26, 2016, to correct the description of the predation risk faced by the fiscals.