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.