5. Sources of Water and Salts
a) Food
Salts and water may be obtained from the food , th e adeq uacy of which dep ends
on the diet. A succulent herbi vorous diet may supply a bird with all of th e wat er
and salts that it requires, and carnivorous or insectivous fare may also contribute
substant ially to its overall requirements. Many bird s subsist on seed s, which when
air dried, contain only about 10% preformed w ater, so that such sp ecies ma y be
particularly dependent on drinking water. Water is formed as a result of metabolism
of fat s, carbohydrates and proteins in the diet so that the se ma y be important com -
ponents of the water intake. CADE and DYBAS (1962) hav e calculated that commercial bird seed y ields metabolic water equivalent to about 45 % of its weight.
Bird s gain a little more water from metabolism of proteins than mammals, reflecting
th e ultimate formation of uric acid instead of ure a; they produce 0.499 g of water
for each gram of protein metabolized compared to 0.396 g in mammals. There is
little or no information about the salt balance in wild birds, but presum ably herbivorous species are subj ect to the same deficiencies as mammals that live in regions
where the sod ium content of the soil is low. Seeds have a ver y low sodium content
compared to the roots , leaves and stems of plants, so that granivorous birds ma y
be expected normally to subsist on much less sodium than other species . The implications of this in the renal and adr enocortical ph ysiology of such birds does not
appear to have been explored.
b) Drinking
Most birds depend on drinking water in order to maintain a positive water balanc e.
The amount of fresh water co nsum ed for this purpose is inversely related to th e
body weight, and in some instances may be equivalent to 100 % of the body wei ght
in a da y (BARTHOLOM EWand CADE, 1963). Some bird s can consume lar ge volumes
of w ater ver y rapidly ; th e mourning dove can drink w ater equivalent to 14% of
its body weight in 30 minutes (SMYTH and BARTHOLOM EW, 1966), a procedure that
may reduce the risk of being prey at water holes, and also reflects the birds' ability
to tra vel, and survive, for extended periods between drinks .
.
Birds in th e laboratory can oft en be persuaded to drink brackish-saline solutions or even sea-water. The concentrations of such solutions that are accepta ble,
and the volumes imbibed, var y a great deal from sp ecies to species and from individual to ind ividual. The stim ulus to drink su ch solutions is simply withdrawal
of fresh water and a diet of relati vely dr y food which leave th e bi~ds no other choice.
Such an alternative ma y be reasonable or unreasonable, depending on the abil ity
to excrete the excess salt. Th e fr equency with which birds actually drink brackish
or sea-water in their natural surro undings is uncertain. Marine birds, especially
those that eat invert ebrates, must involuntarily tak e in some sea- water during feed -
ing. Terrestrial birds in som e area s, such as salt marsh es, have ample supplies of
brackish w ater availabl e, but th e extent to which the se are utilized is unknown .
Even the savannah sparrow, Passerculus sandw ich ensis beldingi, which lives around
salt marshes in California, and can be persuaded to drink salt solutions in th e lab128
a) Food
Salts and water may be obtained from the food , th e adeq uacy of which dep ends
on the diet. A succulent herbi vorous diet may supply a bird with all of th e wat er
and salts that it requires, and carnivorous or insectivous fare may also contribute
substant ially to its overall requirements. Many bird s subsist on seed s, which when
air dried, contain only about 10% preformed w ater, so that such sp ecies ma y be
particularly dependent on drinking water. Water is formed as a result of metabolism
of fat s, carbohydrates and proteins in the diet so that the se ma y be important com -
ponents of the water intake. CADE and DYBAS (1962) hav e calculated that commercial bird seed y ields metabolic water equivalent to about 45 % of its weight.
Bird s gain a little more water from metabolism of proteins than mammals, reflecting
th e ultimate formation of uric acid instead of ure a; they produce 0.499 g of water
for each gram of protein metabolized compared to 0.396 g in mammals. There is
little or no information about the salt balance in wild birds, but presum ably herbivorous species are subj ect to the same deficiencies as mammals that live in regions
where the sod ium content of the soil is low. Seeds have a ver y low sodium content
compared to the roots , leaves and stems of plants, so that granivorous birds ma y
be expected normally to subsist on much less sodium than other species . The implications of this in the renal and adr enocortical ph ysiology of such birds does not
appear to have been explored.
b) Drinking
Most birds depend on drinking water in order to maintain a positive water balanc e.
The amount of fresh water co nsum ed for this purpose is inversely related to th e
body weight, and in some instances may be equivalent to 100 % of the body wei ght
in a da y (BARTHOLOM EWand CADE, 1963). Some bird s can consume lar ge volumes
of w ater ver y rapidly ; th e mourning dove can drink w ater equivalent to 14% of
its body weight in 30 minutes (SMYTH and BARTHOLOM EW, 1966), a procedure that
may reduce the risk of being prey at water holes, and also reflects the birds' ability
to tra vel, and survive, for extended periods between drinks .
.
Birds in th e laboratory can oft en be persuaded to drink brackish-saline solutions or even sea-water. The concentrations of such solutions that are accepta ble,
and the volumes imbibed, var y a great deal from sp ecies to species and from individual to ind ividual. The stim ulus to drink su ch solutions is simply withdrawal
of fresh water and a diet of relati vely dr y food which leave th e bi~ds no other choice.
Such an alternative ma y be reasonable or unreasonable, depending on the abil ity
to excrete the excess salt. Th e fr equency with which birds actually drink brackish
or sea-water in their natural surro undings is uncertain. Marine birds, especially
those that eat invert ebrates, must involuntarily tak e in some sea- water during feed -
ing. Terrestrial birds in som e area s, such as salt marsh es, have ample supplies of
brackish w ater availabl e, but th e extent to which the se are utilized is unknown .
Even the savannah sparrow, Passerculus sandw ich ensis beldingi, which lives around
salt marshes in California, and can be persuaded to drink salt solutions in th e lab128