to drink, the mean sodium concentration in the urine after 24 h fasting was 8.5
m-equiv/l and the standard error was 3.7, indicating that concentrations as low
as 1 m-equiv/l can be attained. The total sodium loss in these ducks (weighing 2.2
kg) was only 1.25 m-equiv/day. Potassium excretion was about 4 times as great,
probably reflecting a release resulting from tissue metabolism due to fasting . The
low rate of sodium excretion is achieved by tubular reabsorption of 99.8 ±0.1%
of the sodium that is filtered at the glomerulus; changes in sodium excretion do
not normally appear to be mediated by changes in the GFR.
HOLMES and his collaborators collected the urine as it was voided from the
cloaca, so that any changes that may have occurred after incubation of the urine
in the posterior regions of the intestine are unknown. As discussed earlier, sodium
can be reabsorbed from the urine after it is regurgitated up into the large intestine.
Absorption of sodium from the prospective faeces presumably occurs simultaneously in the large intestine.
Many birds obtain an excess of salts in their diet. Marine birds, especially those
feeding on invertebrates, must take in substantial quantities of salts with their food.
On Rottnest island near the southwest coast of Australia I have observed ducks
feeding in salt lakes that attain saturation levels of solutes in the summertime. These
lakes contain little life, apart from large numbers of brine shrimps, Artemia salina,
which presumably constitute a very salty part of the ducks' diet.
b) Nasal salt glands
When excess salts are ingested by birds they may be excreted by the kidneys, and
in many species also by the nasal salt glands. The kidneys of birds usually can only
form a urine with a moderate salt concentration, so that they have a relatively limited capacity to excrete large quantities of such solutes and still conserve water.
SCHMIDT-NIELSEN and FANGE (1958) found that when pelicans were injected with
solutions of sodium chloride, only 20% of the total salt excreted appeared in the
urine, the rest being in the secretions of the nasal glands. Similarly, HOLMES and
PHILLIPS (1964) found that while potassium could be excreted in relatively substantial amounts by the kidneys, only about 1% of an ingested load of sodium chloride appeared in the urine. It should be recalled, however, that most terrestrial birds
lack a functional nasal salt gland, and so are dependent on their kidneys for excretion of any excess they may obtain in their diet. The concentrations of sodium
and chloride which may be attained in the secretions of the kidneys and nasal glands
are given in Table 4.2.
The size of the nasal salt glands in birds is related to their feeding habits. If
young gulls, Larus glaucescens, or ducklings are regularly given salt solutions to
drink, the nasal glands hypertrophy compared to those birds provided with fresh
water to drink (HOLMES, PHILLIPS, and BUTLER, 1961; SCHMIDT-NIELSEN and
KIM, 1964). Birds living in marine environments have larger nasal glands than those
species which habitually obtain fresh water to drink (TECHNAU, 1936). STAALAND
(1967) compared the sizes of the nasal glands in a number of charadriiforme birds.
He found that in species that rarely live in marine situations, like the snipe, Capella
121
m-equiv/l and the standard error was 3.7, indicating that concentrations as low
as 1 m-equiv/l can be attained. The total sodium loss in these ducks (weighing 2.2
kg) was only 1.25 m-equiv/day. Potassium excretion was about 4 times as great,
probably reflecting a release resulting from tissue metabolism due to fasting . The
low rate of sodium excretion is achieved by tubular reabsorption of 99.8 ±0.1%
of the sodium that is filtered at the glomerulus; changes in sodium excretion do
not normally appear to be mediated by changes in the GFR.
HOLMES and his collaborators collected the urine as it was voided from the
cloaca, so that any changes that may have occurred after incubation of the urine
in the posterior regions of the intestine are unknown. As discussed earlier, sodium
can be reabsorbed from the urine after it is regurgitated up into the large intestine.
Absorption of sodium from the prospective faeces presumably occurs simultaneously in the large intestine.
Many birds obtain an excess of salts in their diet. Marine birds, especially those
feeding on invertebrates, must take in substantial quantities of salts with their food.
On Rottnest island near the southwest coast of Australia I have observed ducks
feeding in salt lakes that attain saturation levels of solutes in the summertime. These
lakes contain little life, apart from large numbers of brine shrimps, Artemia salina,
which presumably constitute a very salty part of the ducks' diet.
b) Nasal salt glands
When excess salts are ingested by birds they may be excreted by the kidneys, and
in many species also by the nasal salt glands. The kidneys of birds usually can only
form a urine with a moderate salt concentration, so that they have a relatively limited capacity to excrete large quantities of such solutes and still conserve water.
SCHMIDT-NIELSEN and FANGE (1958) found that when pelicans were injected with
solutions of sodium chloride, only 20% of the total salt excreted appeared in the
urine, the rest being in the secretions of the nasal glands. Similarly, HOLMES and
PHILLIPS (1964) found that while potassium could be excreted in relatively substantial amounts by the kidneys, only about 1% of an ingested load of sodium chloride appeared in the urine. It should be recalled, however, that most terrestrial birds
lack a functional nasal salt gland, and so are dependent on their kidneys for excretion of any excess they may obtain in their diet. The concentrations of sodium
and chloride which may be attained in the secretions of the kidneys and nasal glands
are given in Table 4.2.
The size of the nasal salt glands in birds is related to their feeding habits. If
young gulls, Larus glaucescens, or ducklings are regularly given salt solutions to
drink, the nasal glands hypertrophy compared to those birds provided with fresh
water to drink (HOLMES, PHILLIPS, and BUTLER, 1961; SCHMIDT-NIELSEN and
KIM, 1964). Birds living in marine environments have larger nasal glands than those
species which habitually obtain fresh water to drink (TECHNAU, 1936). STAALAND
(1967) compared the sizes of the nasal glands in a number of charadriiforme birds.
He found that in species that rarely live in marine situations, like the snipe, Capella
121