As for all living things, water is essential to the survival of birds. All birds lose water to their environment by evaporation from the moist lining of the lungs as they breathe, and although they lack specialized sweat glands, birds also lose water through the skin. Water is also lost when waste products are excreted. The rate of water loss depends on several factors. A key one, of course, is weather. In hot, dry conditions water loss is high, as birds use evaporation to cool themselves. Another factor is the size of the bird. Water loss, like heat loss, is relatively higher in small birds compared with large, because of the greater surface area in relation to volume in small birds. A bird's pattern of activity -- how much it flies as opposed to rests, for example -- will also influence the rate of water loss.

Most birds drink to make up for the loss, and do so by dipping the bill and then tipping the head back to let the water run down into the throat to be swallowed, which may explain the apparent "sky-pointing" behavior in long-billed species such as curlews. Many small birds use dewdrops as a source of water. Pelicans sometimes drink by holding their beaks open in the rain. Northern Fulmars, doves, and pigeons drink more like horses, immersing the bill and sucking up the water.

Not all land birds need to drink water, however. Hummingbirds, with their largely liquid diet of nectar, normally face a problem of flooding rather than dehydration. Birds of and areas may either go very long periods without drinking or never drink at all. They manage this in part by manufacturing water, as we all do, in the process of "burning" their food (cellular respiration). They also obtain water from their, food (even dry seeds contain some), and they conserve water.

The main function of a bird's kidneys is to remove from the blood the nitrogen-containing wastes formed during the breakdown of proteins -- and to do so while maintaining the proper balance of water, salts, and other materials in the body. In and environments birds can remove these wastes while passing very little water in the urine.

Most mammals excrete these wastes largely in the form of urea, a rather poisonous compound that must be diluted with considerable water. Birds excrete uric acid, which does not dissolve easily in water, is relatively nontoxic, and can be voided nearly dry. Birds, however, must use much more energy to produce the uric acid than mammals do to produce urea. Thus they pay a price for their efficient water retention. Like birds, reptiles excrete uric acid and also pay a high energetic price. Presumably the excretion of uric acid originally evolved in both groups to permit the laying of terrestrial eggs. Fish and amphibian eggs can pass water-soluble nitrogen compounds, ammonia and urea, into the water in which they are bathed. Reptile and bird embryos must store their nitrogenous wastes inside the egg, and to keep from poisoning themselves, they manufacture uric acid. With vast new terrestrial environments thus opened to reptiles and their avian descendants, the energetic cost of uric acid production by the embryo proved a bargain. Evolution then simply coopted its "invention" for adult birds and reptiles, as well.

SEE: Temperature Regulation and Behavior; Hummingbirds, Nectar, and Water; Metabolism; Eggs and Their Evolution.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.