laboratory at 21
0
• Budgerygahs also have a remarkable ability to survive without
drinking water; a group of five domestic birds kept by CADE and DYBAS (1962)
lived at least 38 days at 30
0 and substantially maintained their body weight. Small
birds of this size usually die within 2 or 3 days if deprived of water, so that these
two Australian species are particularly interesting in this respect.
As previously described, evaporation is the largest channel for water loss in
birds, especially those weighing less than about 40 g. At an equitable 25
0 this can
be equivalent to as much as 35% of the body weight in a day (BARTHOLOMEW and
CADE, 1963). How then do these two small birds attain a positive water balance
in such circumstances? The laboratory diets of seeds contain about 10% preformed
water while that which can be produced by metabolizing the seeds is equivalent
to about 45% of their weight. The diet in the field probably yields a similar amount
of water, though I have found that budgerygahs in captivity will also eat (with relish!) more succulent food, such as lettuce and fruit. These birds in their natural
habitat may do the same, especially when drinking water is not available. When
provided with a granivorous diet, zebra finches normally drink water equivalent
to 25% of their body weight in a day, while in the same time budgerygahs imbibe
only 5 or 6% of their weight (CALDER, 1964; CADE et al., 1965; CADE and DYBAS,
1962).
When deprived of such drinking water, buctgerygahs and zebra finches can,
as we have seen, survive in the laboratory for extended periods of time, apparently
deriving sufficient water for their basal sustenance from the seeds the y eat. The
available knowledge about the water balance in these birds is summarized in Table
4.4. The conclusion from these results is that such birds, even at 20
0
, experience
a net water loss, but it mu st be emphasized that the values for pulmocutaneous
water loss were collected when the animals were breathing dry air. When the birds
are breathing more humid air, as they do in laboratory cages, the evaporative water
loss would be substantially less. It is apparent that these birds when deprived of
water, make certain adjustments, as the combined urinary and faecal water losses
are reduced. KRAG and SKADHAUGE (1970) have recently made direct measurements
of the ureteral urine in budgerygahs under both normal conditions and during dehydration. The flow and concentration of this urine was similar in both circumstances, so that the birds are normally in a 'fairly antidiuretic state'. The decreased
total excretory water losses previously observed thus are presumably the result of
a decreased faecal loss and /or an increased rate of water reabsorption from the
cloaca of the dehydrated budgies. The evaporative water loss declines, especially
at high ambient temperatures. The reasons for the latter are not clear, but they are
not the result of a reduction in the resting level of oxygen consumption although
they could reflect differences in activity. The losses of water from the respiratory
tract are reduced in relation to the amount of oxygen consumed; this could be the
result of a more efficient extraction of oxygen from the inspired air, or a reduction
in the temperature of the expired air with a resulting decrease in its content of water
vapour. In the budgerygah such savings of evaporative water are greater at ambient
temperatures over about 36
0
, when they result in an impaired ability to regulate
the body temperature. These birds, when deprived of water reduce their activity,
and so conserve water that would be lost during the added respiratory activity.
Both the budgerygah and the zebra finch do not start to utilize evaporation of water
131
0
• Budgerygahs also have a remarkable ability to survive without
drinking water; a group of five domestic birds kept by CADE and DYBAS (1962)
lived at least 38 days at 30
0 and substantially maintained their body weight. Small
birds of this size usually die within 2 or 3 days if deprived of water, so that these
two Australian species are particularly interesting in this respect.
As previously described, evaporation is the largest channel for water loss in
birds, especially those weighing less than about 40 g. At an equitable 25
0 this can
be equivalent to as much as 35% of the body weight in a day (BARTHOLOMEW and
CADE, 1963). How then do these two small birds attain a positive water balance
in such circumstances? The laboratory diets of seeds contain about 10% preformed
water while that which can be produced by metabolizing the seeds is equivalent
to about 45% of their weight. The diet in the field probably yields a similar amount
of water, though I have found that budgerygahs in captivity will also eat (with relish!) more succulent food, such as lettuce and fruit. These birds in their natural
habitat may do the same, especially when drinking water is not available. When
provided with a granivorous diet, zebra finches normally drink water equivalent
to 25% of their body weight in a day, while in the same time budgerygahs imbibe
only 5 or 6% of their weight (CALDER, 1964; CADE et al., 1965; CADE and DYBAS,
1962).
When deprived of such drinking water, buctgerygahs and zebra finches can,
as we have seen, survive in the laboratory for extended periods of time, apparently
deriving sufficient water for their basal sustenance from the seeds the y eat. The
available knowledge about the water balance in these birds is summarized in Table
4.4. The conclusion from these results is that such birds, even at 20
0
, experience
a net water loss, but it mu st be emphasized that the values for pulmocutaneous
water loss were collected when the animals were breathing dry air. When the birds
are breathing more humid air, as they do in laboratory cages, the evaporative water
loss would be substantially less. It is apparent that these birds when deprived of
water, make certain adjustments, as the combined urinary and faecal water losses
are reduced. KRAG and SKADHAUGE (1970) have recently made direct measurements
of the ureteral urine in budgerygahs under both normal conditions and during dehydration. The flow and concentration of this urine was similar in both circumstances, so that the birds are normally in a 'fairly antidiuretic state'. The decreased
total excretory water losses previously observed thus are presumably the result of
a decreased faecal loss and /or an increased rate of water reabsorption from the
cloaca of the dehydrated budgies. The evaporative water loss declines, especially
at high ambient temperatures. The reasons for the latter are not clear, but they are
not the result of a reduction in the resting level of oxygen consumption although
they could reflect differences in activity. The losses of water from the respiratory
tract are reduced in relation to the amount of oxygen consumed; this could be the
result of a more efficient extraction of oxygen from the inspired air, or a reduction
in the temperature of the expired air with a resulting decrease in its content of water
vapour. In the budgerygah such savings of evaporative water are greater at ambient
temperatures over about 36
0
, when they result in an impaired ability to regulate
the body temperature. These birds, when deprived of water reduce their activity,
and so conserve water that would be lost during the added respiratory activity.
Both the budgerygah and the zebra finch do not start to utilize evaporation of water
131