Birds — evaporative cooling

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One searingly hot summer afternoon, I spotted a thrasher standing quite still on the ground in the shade of a small tree. The thrasher’s long curved bill was open and its wings slightly spread. At first I thought that it was sick. But then I noticed a plump white-winged dove perched on a branch overhead. It too held its mouth wide open; as I watched, I could see the skin of its throat pulsating rapidly.

Neither bird was sick. Like me, they were simply hot. Unlike people however, birds lack sweat glands and so cannot sweat to cool themselves down. But birds call on a variety of innovative techniques to beat the heat. Black vultures and wood storks, for instance, use a highly practical, if not pretty method: they defecate on their unfeathered feet and legs. As the moisture in the excretia evaporates, the bare skin cools quickly, sucking heat from the bird¹s body. Vultures and other large soaring birds also cool themselves by riding thermals to several thousand feet up in the atmosphere where the air may be 50 degrees cooler than on the ground.

Most birds use more prosaic cooling methods, such as seeking shade, bathing and/or reducing their activity - a bird in flight produces from 9 to 15 times as much heat as a resting bird. They also simply reverse their heating tactics: Instead of fluffing up their feathers, they compress their plumage to retain as little body heat as possible. And they increase circulation to unfeathered parts that will radiate heat from their blood to the outside air.

When air temperatures rise over 100 degrees, many birds - like the thrasher that I watched - pant, stepping up their breathing rate to expel hot, moist air from inside their bodies. The influx of dry outside air also cools the bird evaporatively from within by vaporizing water in its lungs and its air sacs, a system of balloon-like extensions of the lungs that fill most of the extra space in a bird¹s body, including some of its bones. Most birds can dissipate about half of their resting heat production by panting.

In addition to panting, some birds - like the perched white-winged dove - pulse the skin of their throat in and out, and at the same time, increase the blood flow to their throat skin. Like a car radiator cooling the hot water from the engine, the fluttering skin radiates the heat of the bird¹s blood to the air.

Sweat began to trickle down my back as I stood watching the two birds. After a moment, I walked on, headed for the air-conditioned campus library. As I pulled open the door, releasing a gush of cool air, I looked back. The thrasher and the white-winged dove sat motionless in the shade, their mouths open, panting.