Humans have about 2.6 million sweat glands for cooling by evaporation of water and heat from the skin. Women have more sweat glands than men but sweat less because their skin temperatures are higher and more heat is lost per drop of sweat. Birds have no sweat glands so they depend largely upon evaporative water loss from their skin, lungs and air sacs, but to avoid carrying a heavy load of water their physiological makeup is such that evaporative water loss is minimized. Birds can also acclimate somewhat to humidity; sparrows experimentally kept in a dry environment for three weeks lost 36 percent less water from their skin than those birds kept in a moist environment.

It appears that, overall, heat stress or hyperthermia, elevated body temperature, is more severe than cold stress (hypothermia.) Overheating is a common problem faced by poultry producers, as too high an ambient temperature leads to a loss of weight, poor quality meat, and lower egg weight. At a temperature of 104⁰ F (40⁰ C), confined chickens will die in about three hours. The U.K. actually has a protocol for killing chickens in order to control a disease epidemic by the use of hyperthermia. But overheating is also a concern for wild birds, especially with increasing global temperatures. In January of 2009, thousands of birds, mainly parrots, died in Carnarvon, Western Australia, when the temperature reached 113⁰ F (45⁰ C) and the following year 208 Carnaby’s Black Cockatoos, an endangered species, were found dead on the south coast when the temperature hit 118⁰ F (48⁰ C). A small bird can lose five percent of its weight every hour at that temperature, rapidly becoming dehydrated; death can occur in a matter of hours.

Desert-dwelling birds perhaps face the greatest challenges. Much of the Arabian Peninsula is a very dry desert where more than a year can pass without rainfall and the average summer humidity might reach 15 percent. The Hoopoe and Crested Larks of the Arabian Desert confront air temperatures of over 113⁰F (45⁰C) and ground surface temperatures as high as 140⁰F (60⁰C). There is a paucity of both plant species and  individual plants so obtaining sufficient food is a challenge. How do the larks survive? They are capable of lowering their basal metabolic rates by about 40 percent and decrease the loss of water from their skin and respiratory systems by almost 30 percent. During the hottest times of the day, the birds often hole up in the burrows of Egyptian Spiny-tailed Lizards. While ensconced in these warrens the Hoopoe Larks scrape away the top layer of sand and prostrate themselves with their neck and chest against the ground. Presumably the birds are conducting heat away from their body without losing much water from their lungs. Crouching in a lizard burrow allows the birds to reduce their potential water loss by 81 percent.