oratory, has 'never' been observed 'drinking under natural conditions ... (and) ...
depend(s) to a large extent on the water which condenses on the marsh vegetation
from the frequent coastal fogs' (POULSEN and BARTHOLOMEW, 1962). In special
circumstances, it is conceivable that birds may utilize their physiological ability
to drink saline solutions, and this may be useful in 'osmotic' emergencies, like those
that occur in laboratories.
The salt that is taken in with the drinking water may be excreted by either the
kidneys or, especially in marine birds, by the nasal salt glands . The ability to gain
free water from an ingested saline solution depends on the ability to produce urine
or a nasal gland fluid with a higher concentration than that taken by drinking.The
maximum concentrations that can be achieved by the kidney and nasal glands vary
in different species (Table 4.2). It should be remembered that such concentrations
are often attained only in special circumstances, and cannot necessarily be maintained during the secretion of large volumes of fluid. Sea-water usually has a concentration equivalent to a 560 m-equiv/I (3.3%) sodium chloride solution, so that
a gain of free water from drinking such solutions is feasible, especially in many
species with functional nasal salt glands. Such birds are nearly all marine species.
Nasal salt glands have also been found in the ostrich, Struthio camelus, and the
desert partridge, Ammoperdix heyi, though their physiological importance in these
birds is not yet clear (SCHMIDT-NIELSEN et al., 1963).
6. Reproduction, Migration and Osmoregulation
Reproduction results in an increased need for water and salts for the formation
of eggs, and in many instances for the maintenance of the young. As described later,
certain birds from dry areas, such as the budgerygah and zebra finch, do not breed
regularly but await a stimulus, or signal, which is associated with rainfall (MARSHALL, 1961). Endocrines play an important role in the initiation and regulation
of reproduction, but the manner in which rain can influence this is not clear. The
effects may conceivably be indirect and result from an improved nutritional state,
due to an increase in the supply of food , or, as shown by MARSHALL and DISNEY
(1957) in the diochs, Quelea quelea, from Tanzania, by the sight of green grass .
Other stimuli could include humidity or an optimal state of body hydration (CADE,
TOBIN, and GOLD 1965).
Many birds are relatively self-sufficient when they hatch, and fend for food
and water by themselves. Others may first spend a period in the nest where they
are tended by adult birds. Food may be collected by the adults and provided
directly to the young, but in other instances it is first swallowed and regurgitated
in a moistened and partially digested condition. Columbiforme birds, such as
pigeons and doves, secrete a special fluid into their crop which they feed to the
young. This secretion, called 'pigeons milk', appears to be under the control of
the adenohypophysial hormone prolactin, as injections of this stimulate growth
and secretion of the special cells concerned in this process (RIDDLE and BRAUCHER,
1931). This hormone, or a related natural analogue, is concerned with the control of secretion of milk in mammals and the osmoregulation in certain fishes. Birds
that feed their young with a secretion from their crop, or with regurgitated and
129
depend(s) to a large extent on the water which condenses on the marsh vegetation
from the frequent coastal fogs' (POULSEN and BARTHOLOMEW, 1962). In special
circumstances, it is conceivable that birds may utilize their physiological ability
to drink saline solutions, and this may be useful in 'osmotic' emergencies, like those
that occur in laboratories.
The salt that is taken in with the drinking water may be excreted by either the
kidneys or, especially in marine birds, by the nasal salt glands . The ability to gain
free water from an ingested saline solution depends on the ability to produce urine
or a nasal gland fluid with a higher concentration than that taken by drinking.The
maximum concentrations that can be achieved by the kidney and nasal glands vary
in different species (Table 4.2). It should be remembered that such concentrations
are often attained only in special circumstances, and cannot necessarily be maintained during the secretion of large volumes of fluid. Sea-water usually has a concentration equivalent to a 560 m-equiv/I (3.3%) sodium chloride solution, so that
a gain of free water from drinking such solutions is feasible, especially in many
species with functional nasal salt glands. Such birds are nearly all marine species.
Nasal salt glands have also been found in the ostrich, Struthio camelus, and the
desert partridge, Ammoperdix heyi, though their physiological importance in these
birds is not yet clear (SCHMIDT-NIELSEN et al., 1963).
6. Reproduction, Migration and Osmoregulation
Reproduction results in an increased need for water and salts for the formation
of eggs, and in many instances for the maintenance of the young. As described later,
certain birds from dry areas, such as the budgerygah and zebra finch, do not breed
regularly but await a stimulus, or signal, which is associated with rainfall (MARSHALL, 1961). Endocrines play an important role in the initiation and regulation
of reproduction, but the manner in which rain can influence this is not clear. The
effects may conceivably be indirect and result from an improved nutritional state,
due to an increase in the supply of food , or, as shown by MARSHALL and DISNEY
(1957) in the diochs, Quelea quelea, from Tanzania, by the sight of green grass .
Other stimuli could include humidity or an optimal state of body hydration (CADE,
TOBIN, and GOLD 1965).
Many birds are relatively self-sufficient when they hatch, and fend for food
and water by themselves. Others may first spend a period in the nest where they
are tended by adult birds. Food may be collected by the adults and provided
directly to the young, but in other instances it is first swallowed and regurgitated
in a moistened and partially digested condition. Columbiforme birds, such as
pigeons and doves, secrete a special fluid into their crop which they feed to the
young. This secretion, called 'pigeons milk', appears to be under the control of
the adenohypophysial hormone prolactin, as injections of this stimulate growth
and secretion of the special cells concerned in this process (RIDDLE and BRAUCHER,
1931). This hormone, or a related natural analogue, is concerned with the control of secretion of milk in mammals and the osmoregulation in certain fishes. Birds
that feed their young with a secretion from their crop, or with regurgitated and
129