lower than those in mammals. Most species so far examined can concentrate their
urine to give maximum urine to plasma osmotic concentration ratio of about 2.
Even a desert bird like the roadrunner, Geococcyx californianus, or a sea bird like
the pelican, Pelecanus erytbrorbyncbos, when deprived of water, only produces
a urine with a concentration of about 700 m-osmole/l (CALDER and BENTLEY,
1967). A notable exception is the savannah sparrow, Passerculus sandwichensisbeldingi, which inhabits saltmarshes in California, and wh ich, when given saline solutions to drink, can produce a urine with a concentration of 2000 m-osmole/l,
(POULSEN and BARTHOLOMEW, 1962). Many birds are not completely dependent
on the kidneys for electrolyte excretion, as the nasal salt glands can secrete large
amounts of highly concentrated fluids . The need for metabolic nitrogen excretion
accounts for a substantial part of the renal osmotic water, and in birds such excretion occurs largel y in the form of uric acid . Ducks excrete 54 % of their catabolic
nitrogen as uric acid, 29% as ammonia and less than 2% as urea (STEWART, HOLM ES,
and FLETCHER, 1969). It will be recalled that mammals form principally urea . Uric
acid, in contrast to urea, is poorly soluble in water and is precipitated in th e urine
to form a milky suspension in which state it exerts negligible osmotic pressure.
One gram of nitrogen, when excreted by a bird, requires only about 10 ml of
urine for its excretion while the same amount of nitrogen in the form of urea requires 50 ml for its' excretion in a man (H. SMITH, 1951).
a) Renal Water Conservation and the Neurohypophysis. After water is administered to the domestic fowl there is an increase in urine flow, accompanied by a
decrease in its concentration from about 500 m-osmole/l to 80 m-osmole/l (DICKER
and HASLAM, 1966). The glomerular filtration rate (GFR), as reflected by the endogenous creatinine clearance, markedly increases and this is closely related to the
elevated urine volume. KORR (1939) also observed an increased GFR in hydrated
chickens . SKADHAUGE and B. SCHMIDT-NIELSEN (1967) found that the urine increased in volume, and decreased in concentration following hydration of roosters,
but they observed onl y a small (23 %) increase in the GFR. The reasons for these
differences in glomerular behaviour are not clear. The latter authors used inulin
clearance to determine GFR and, while this is a more accurate method of measuring
the GFR, the magnitude of the differences is still difficult to und erstand. The GFR
of hydrated budgerygahs is about one-third greater than when they are dehydrated,
but as the urine flow is nearl y four tim es larger, tubular wate r reabsorption is predominant in this bird also (KRAG and SKADHAUGE pers. comm .). The exact role
of changes in the GFR in alterations of the urine volume in the birds is uncertain.
All th e results, nevertheless, indicate that changes in the rates of water reabsorption
from the renal tubules are of major importance. The GFR in mammals is more
labile in some species, like the dog, than in others such as man, and experiments
on more avian species may elucidate the role of the glomerulus in such processes.
Dehydration (SKADHAUGE and B. SCHMIDT-NIELSEN, 1967) or infusion of hypertonic sodium chloride solutions (DANTZLER, 1966) results in a decreased urine
flow in the domestic fowl and such treatments are accompanied by decreases of the
GFR as also seen in reptiles. D ANTZLER infused 6 % sodium chloride solutions into
chickens and observed a 40% reduction in GFR (inulin clearance). This was found
to result from a decreased number of functioning glomeruli (glomerular int er116
urine to give maximum urine to plasma osmotic concentration ratio of about 2.
Even a desert bird like the roadrunner, Geococcyx californianus, or a sea bird like
the pelican, Pelecanus erytbrorbyncbos, when deprived of water, only produces
a urine with a concentration of about 700 m-osmole/l (CALDER and BENTLEY,
1967). A notable exception is the savannah sparrow, Passerculus sandwichensisbeldingi, which inhabits saltmarshes in California, and wh ich, when given saline solutions to drink, can produce a urine with a concentration of 2000 m-osmole/l,
(POULSEN and BARTHOLOMEW, 1962). Many birds are not completely dependent
on the kidneys for electrolyte excretion, as the nasal salt glands can secrete large
amounts of highly concentrated fluids . The need for metabolic nitrogen excretion
accounts for a substantial part of the renal osmotic water, and in birds such excretion occurs largel y in the form of uric acid . Ducks excrete 54 % of their catabolic
nitrogen as uric acid, 29% as ammonia and less than 2% as urea (STEWART, HOLM ES,
and FLETCHER, 1969). It will be recalled that mammals form principally urea . Uric
acid, in contrast to urea, is poorly soluble in water and is precipitated in th e urine
to form a milky suspension in which state it exerts negligible osmotic pressure.
One gram of nitrogen, when excreted by a bird, requires only about 10 ml of
urine for its excretion while the same amount of nitrogen in the form of urea requires 50 ml for its' excretion in a man (H. SMITH, 1951).
a) Renal Water Conservation and the Neurohypophysis. After water is administered to the domestic fowl there is an increase in urine flow, accompanied by a
decrease in its concentration from about 500 m-osmole/l to 80 m-osmole/l (DICKER
and HASLAM, 1966). The glomerular filtration rate (GFR), as reflected by the endogenous creatinine clearance, markedly increases and this is closely related to the
elevated urine volume. KORR (1939) also observed an increased GFR in hydrated
chickens . SKADHAUGE and B. SCHMIDT-NIELSEN (1967) found that the urine increased in volume, and decreased in concentration following hydration of roosters,
but they observed onl y a small (23 %) increase in the GFR. The reasons for these
differences in glomerular behaviour are not clear. The latter authors used inulin
clearance to determine GFR and, while this is a more accurate method of measuring
the GFR, the magnitude of the differences is still difficult to und erstand. The GFR
of hydrated budgerygahs is about one-third greater than when they are dehydrated,
but as the urine flow is nearl y four tim es larger, tubular wate r reabsorption is predominant in this bird also (KRAG and SKADHAUGE pers. comm .). The exact role
of changes in the GFR in alterations of the urine volume in the birds is uncertain.
All th e results, nevertheless, indicate that changes in the rates of water reabsorption
from the renal tubules are of major importance. The GFR in mammals is more
labile in some species, like the dog, than in others such as man, and experiments
on more avian species may elucidate the role of the glomerulus in such processes.
Dehydration (SKADHAUGE and B. SCHMIDT-NIELSEN, 1967) or infusion of hypertonic sodium chloride solutions (DANTZLER, 1966) results in a decreased urine
flow in the domestic fowl and such treatments are accompanied by decreases of the
GFR as also seen in reptiles. D ANTZLER infused 6 % sodium chloride solutions into
chickens and observed a 40% reduction in GFR (inulin clearance). This was found
to result from a decreased number of functioning glomeruli (glomerular int er116