Few birds can survive indefinitely on a diet of dry food with no water to drink.
Most birds progressively lose weight on such a regimen, and this substantially represents a loss of body water. Death ultimately occurs, but the loss that is fatal varies
in different species. The California quail dies after losing 50% of its body weight,
the mourning dove after a loss of 37%, while the house finch cannot survive a loss
of 15% in weight (BARTHOLOMEW and CADE, 1963). The reasons for these differences are not clear but the y may be related to the rates of deh ydration, for the house
finch loses water five times as fast as the California quail and the mourning do ve
at an intermediate rate. Measurements of the concentrations of the body fluids at
death would indicate whether or not these species have different tolerances to
elevated osmotic pressures in their tissues.
a) Evaporative water loss
Relative evaporative water loss in birds is inversely related to their body weight
(Fig. 4.1), an observation that substantially reflects differences in the rates of their
metabolism, and surface area to body weight ratios. Sweat glands are absent in
birds, and it is generally assumed that evaporative water losses take place predominantly from the respiratory tract. The exact magnitude of the cutaneous water
loss is unknown, and the absence of sweat glands per se does not indicate that water
loss through the skin does not occur. The laboratory rat has no sweat glands, but
still loses substantial amounts of water through the skin.
Water loss occurs from the respiratory tract, as an unavoidable consequence
of the exchange of oxygen and carbon dioxide. The high body temperature of birds
potentiates such losses, as the expired air contains more water vapour than it would
do if saturated at lower temperatures. The temperature of the expired air has, ho wever, not been measured and it is possible that it may be reduced by a countercurrent heat exchange in the nasal passages as seen in rodents by JACKSO N and
SCHMIDT-NIELSEN (1964) . Water loss in desert rodents is only about 0.55 mg per
ml of oxygen consumed while in man the loss from the respiratory tract is 0.9
mg per ml of oxygen used (SCHMIDT-NIELSEN, 1964 a). Such water loss in birds
is generally higher than that in mammals . The budgerygah, Melopsittacus undulatus, which lives in the desert areas of Australia, is a bird of similar size to the
kangaroo rat but loses water at nearly twice the rate : 0.94 mg per ml oxygen used
(GREENWALD, STONE, and CADE, 1967). DAWSON and SCHMIDT-NIELSEN (1964)
conclude from the availabl e information that such evaporative losses in birds range
from 0.9 to 2.1 mg water per ml of oxygen used and may be even greater.
When the environmental temperature rises, the evaporative water loss is increased, and as the normal body temperature is approached, this may be physiologically accelerated in response to the need for temperature regulation. The need
for such adjustments is related to the balance of metabolic heat production by the
bird, and the rate at which this is dissipated to the external environment. Such exchanges are dependent on the difference between the temperature of the animal
and the environment, which affects the rate and direction of dissipation of heat
by radiation, conduction and convection. The high body temperature of birds will
favour such heat loss, more than in animals with a lower body temperature that
113
Most birds progressively lose weight on such a regimen, and this substantially represents a loss of body water. Death ultimately occurs, but the loss that is fatal varies
in different species. The California quail dies after losing 50% of its body weight,
the mourning dove after a loss of 37%, while the house finch cannot survive a loss
of 15% in weight (BARTHOLOMEW and CADE, 1963). The reasons for these differences are not clear but the y may be related to the rates of deh ydration, for the house
finch loses water five times as fast as the California quail and the mourning do ve
at an intermediate rate. Measurements of the concentrations of the body fluids at
death would indicate whether or not these species have different tolerances to
elevated osmotic pressures in their tissues.
a) Evaporative water loss
Relative evaporative water loss in birds is inversely related to their body weight
(Fig. 4.1), an observation that substantially reflects differences in the rates of their
metabolism, and surface area to body weight ratios. Sweat glands are absent in
birds, and it is generally assumed that evaporative water losses take place predominantly from the respiratory tract. The exact magnitude of the cutaneous water
loss is unknown, and the absence of sweat glands per se does not indicate that water
loss through the skin does not occur. The laboratory rat has no sweat glands, but
still loses substantial amounts of water through the skin.
Water loss occurs from the respiratory tract, as an unavoidable consequence
of the exchange of oxygen and carbon dioxide. The high body temperature of birds
potentiates such losses, as the expired air contains more water vapour than it would
do if saturated at lower temperatures. The temperature of the expired air has, ho wever, not been measured and it is possible that it may be reduced by a countercurrent heat exchange in the nasal passages as seen in rodents by JACKSO N and
SCHMIDT-NIELSEN (1964) . Water loss in desert rodents is only about 0.55 mg per
ml of oxygen consumed while in man the loss from the respiratory tract is 0.9
mg per ml of oxygen used (SCHMIDT-NIELSEN, 1964 a). Such water loss in birds
is generally higher than that in mammals . The budgerygah, Melopsittacus undulatus, which lives in the desert areas of Australia, is a bird of similar size to the
kangaroo rat but loses water at nearly twice the rate : 0.94 mg per ml oxygen used
(GREENWALD, STONE, and CADE, 1967). DAWSON and SCHMIDT-NIELSEN (1964)
conclude from the availabl e information that such evaporative losses in birds range
from 0.9 to 2.1 mg water per ml of oxygen used and may be even greater.
When the environmental temperature rises, the evaporative water loss is increased, and as the normal body temperature is approached, this may be physiologically accelerated in response to the need for temperature regulation. The need
for such adjustments is related to the balance of metabolic heat production by the
bird, and the rate at which this is dissipated to the external environment. Such exchanges are dependent on the difference between the temperature of the animal
and the environment, which affects the rate and direction of dissipation of heat
by radiation, conduction and convection. The high body temperature of birds will
favour such heat loss, more than in animals with a lower body temperature that
113