Dealing With The Heat

Yellow-crested Cockatoo, Cacatua sulphurea

Birds have morphological and physiological adaptations that allow them to respond to a range of temperatures. As the ambient temperature increases, the physiological mechanisms approach their limits and behavioral mechanisms come into play. Every species and  individual in that species has its own thermoneutral zone, providing a major explanation for the geographic distribution of bird species. 

In birds, the thin fat (lipid) layers of the skin minimize water loss in a thermoneutral environment, but also serve as a mechanism for facilitating heat loss during episodes of hyperthermia. Verdins, like many small birds, typically stay within their thermoneutral zone by losing water from their skin. This cutaneous water loss typically accounts for 50- 65% of the bird’s total water loss. As the temperature rises, however, respiratory water loss – from lungs and air sacs – becomes increasingly important. To minimize that problem, the Verdin and Black-tailed Gnatcatcher of the Sonoran Desert install themselves in tree crevices during the hottest part of the day, forgoing foraging activity to decrease their exposure to heat and avoid the water loss they would incur by searching for food. Diamond Doves of the arid deserts of central Australia have little access to water, so water conservation is imperative. They feed in the heat of the day without shade cover but to survive they manage to keep their basal metabolic rate from rising and conserve water by allowing their body temperature to rise, as do camels, but remaining within the TNZ. The more heat that can be stored the less water is needed to dissipate it. When the air cools, a lot less water needs to be used to dispose of the excess body heat.


Songbirds lose 50 percent of their excess heat by cutaneous water loss as long as the bird is sufficiently hydrated. When dehydration becomes an issue, loss of water from the respiratory system comes into play through normal respiration, followed by panting. Panting, the rapid increase in respiratory rate and thus air volume into and out of the lungs, is a major mechanism for heat loss in birds subjected to heat stress. Panting movements may be as much as 16-27 times the normal respiratory rate. Additionally, if the heat load continues to increase, non-passerines may also employ gular flutter. In some birds the gular or neck region of the skin from the bottom of the jaw to the throat is featherless or nearly so. The floor of the mouth and throat vibrates with the use of hyoid bones and muscles, helping to dissipate heat from the skin. It is common in pelicans, cormorants, anhingas, boobies, frigatebirds, and a number of ground-dwelling birds like nighthawks, turkeys, pheasants, and roadrunners. Ornithology researchers measured the flutter rates of White Pelicans at 235-290 per minute, the Double-crested Cormorant at 645 to 730, and the Mourning Dove at 680-735 flutters per minute. Depending on the species, panting and gular fluttering may be synchronized as it is in owls, or uncoordinated as in pelicans and cormorants. The energetic cost of panting is high as is the amount of water lost so the more efficient mechanism of gular flutter may be used in conjunction with panting or instead of it to conserve both energy and water.


 Rock Pigeons in the Negev Desert move among rock formations to seek shade on hot days. The Crowned Plover raises itself on its legs above its nest and spreads its wings to expose more body surface area; the bird is also exposed more to breezes as the wind speed, even a few inches/centimeters above the ground, is greater. The Great Frigatebird, in addition to gular fluttering, erects its feathers, spreads its wings wide or opens them and lets them sag to allow heat loss. The California Spotted Owl pants, erects its feathers and droops its wings to allow heat to dissipate off its body. European Bee-eaters have been observed diving into fish free salt ponds and the Red Sea to cool themselves; one bee-eater was discovered in the stomach of a shark.


Piloerection, the fluffing of feathers, is used both in increasing and decreasing heat loss; the difference is in the position of the feathers. To cool, the body feathers are lifted high enough so that the skin is exposed and cutaneous water loss increased for cooling as the blood flow to the skin is increased. The Spinifex Pigeon of arid Australia lives in an environment where half the year the temperature in the shade exceeds 1000F (380C). It survives by having a low metabolic rate and manages its cutaneous heat loss by the use of an elevated skin temperature and ptiloerection. Great Knots, wintering on the north coast of Australia prepare for their long migratory flight to the arctic by adding layers of fat; being in a warm environment while doing this poses an overheating problem. To avoid heat stress, the birds raise the blackish feathers of their back to reduce the absorption of solar radiation and increase convective and cutaneous cooling as well. An alternative to raising the feathers is to flatten them out as the Curve-billed Thrasher of the southwestern U.S. and Mexico does. Compressing the feathers against the body reduces their insulating value as most of the air spaces are eliminated.


The Turkey Vulture prevents hyperthermia by spreading its wings, extending its neck and head, and engages in a behavior that adds to the bird’s rather unpleasant reputation – it poops on its legs. Urohydrosis, the wetting of the legs by excreting on them, is sort of like spraying your face or arms and letting the evaporating water cool you off. New World vultures, condors, storks, gannets and boobies also engage in this yucky activity.  Urohydrosis is rather rare in the bird world because it requires easy access to drinking water. An interesting problem arose several years ago when a bird bander discovered that several of the Turkey Vultures that he recaptured had injuries to their tarsi (lower legs) due to the accumulation of waste matter on the leg bands. Since that time the U.S. and Canadian banding services no longer allow leg bands on vultures.