Chapter 4
The Birds
Birds are the most cosmopolitan of the vertebrates. The ability to fly confers on
them a mobility that is matched in other vertebrates only by the bats. Some avian
species have, however, lost the ability to fly, or do so to little effect . Flightless birds
include the ostriches, kiwis, cassowaries, emus and rheas, while many other species
like the domestic fowl are aviatorially inept. Some birds, like the penguins and auks,
use their wings for propulsion under water. Birds thus may live a predominantly
terrestrial or marine existence or they may also utilize the skies above such areas.
Geographically, birds are found on the most remote oceanic islands, in dry continental desert regions, and in tropical and temperate areas where water is abundant.
They can indeed live, and thrive, in a variety of osmotic environments.
The osmotic problems of birds are basically like those of mammals; their osmotic anatomy is similar, they occupy the same geographical regions and habitats
and they are also homoiothermic. Their needs for water and salts are thus potentially comparable, but differences in their physiological and morphological characteristics may modify their respective requirements. Most birds are smaller than
mammals, so that they more often experience the difficulties inherent in a large
surface area to body weight ratio. The body temperature in birds is usually in the
region of 40 to 42
0
, which is 3
0 or 4
0 higher than that of most mammals. This
reflects a metabolic rate that is higher than that of mammals, and so modifies heat
exchanges with the environment. Flight results in metabolic burdens which are
completely unfamiliar to mammals (except possibly the bats), and this is reflected
in increased respiratory gas exchange, with an accompanying additional water loss.
The mobility conferred by flying may, however, be a considerable osmotic advantage, as it allows birds to travel rapidly for long distances to suitable feeding and
watering places. This may be a relatively local commuting, or it may take place
between major geographical regions, in which seasonal changes in available food
and water may occur.
The birds exhibit a number of other physiological features, relevant to their
osmoregulation, that they share with either the mammals or their phyletic progenitors the reptiles. Both the birds and mammals, in contrast to the reptiles, can form
a hypertonic urine. However, in birds the maximal concentrations are not as great
as usually seen in mammals, but they conserve additional renal water by converting
most of their catabolic nitrogen to uric acid. Most reptiles are similarly uricotelic.
While in the mammals the kidney is the principal route for excretion of salts, some
birds and reptiles can also secrete such solutes from cephalic 'salt' glands. Birds,
like reptiles, but in contrast to mammals (except the monotremes), are oviparous,
and it can be conjectured that this could influence the pattern of their osmoregulation. The rapid production of a clutch of eggs containing all of the water
and salt necessary for an extended period of embryonic growth would seem to result in a more acute need for water and salt than embryonic development in utero.
Tending and incubating such eggs temporarily restricts the movements of birds
so that breeding can only occur in a place and time of adequate proximal supplies
of food and water.
111
The Birds
Birds are the most cosmopolitan of the vertebrates. The ability to fly confers on
them a mobility that is matched in other vertebrates only by the bats. Some avian
species have, however, lost the ability to fly, or do so to little effect . Flightless birds
include the ostriches, kiwis, cassowaries, emus and rheas, while many other species
like the domestic fowl are aviatorially inept. Some birds, like the penguins and auks,
use their wings for propulsion under water. Birds thus may live a predominantly
terrestrial or marine existence or they may also utilize the skies above such areas.
Geographically, birds are found on the most remote oceanic islands, in dry continental desert regions, and in tropical and temperate areas where water is abundant.
They can indeed live, and thrive, in a variety of osmotic environments.
The osmotic problems of birds are basically like those of mammals; their osmotic anatomy is similar, they occupy the same geographical regions and habitats
and they are also homoiothermic. Their needs for water and salts are thus potentially comparable, but differences in their physiological and morphological characteristics may modify their respective requirements. Most birds are smaller than
mammals, so that they more often experience the difficulties inherent in a large
surface area to body weight ratio. The body temperature in birds is usually in the
region of 40 to 42
0
, which is 3
0 or 4
0 higher than that of most mammals. This
reflects a metabolic rate that is higher than that of mammals, and so modifies heat
exchanges with the environment. Flight results in metabolic burdens which are
completely unfamiliar to mammals (except possibly the bats), and this is reflected
in increased respiratory gas exchange, with an accompanying additional water loss.
The mobility conferred by flying may, however, be a considerable osmotic advantage, as it allows birds to travel rapidly for long distances to suitable feeding and
watering places. This may be a relatively local commuting, or it may take place
between major geographical regions, in which seasonal changes in available food
and water may occur.
The birds exhibit a number of other physiological features, relevant to their
osmoregulation, that they share with either the mammals or their phyletic progenitors the reptiles. Both the birds and mammals, in contrast to the reptiles, can form
a hypertonic urine. However, in birds the maximal concentrations are not as great
as usually seen in mammals, but they conserve additional renal water by converting
most of their catabolic nitrogen to uric acid. Most reptiles are similarly uricotelic.
While in the mammals the kidney is the principal route for excretion of salts, some
birds and reptiles can also secrete such solutes from cephalic 'salt' glands. Birds,
like reptiles, but in contrast to mammals (except the monotremes), are oviparous,
and it can be conjectured that this could influence the pattern of their osmoregulation. The rapid production of a clutch of eggs containing all of the water
and salt necessary for an extended period of embryonic growth would seem to result in a more acute need for water and salt than embryonic development in utero.
Tending and incubating such eggs temporarily restricts the movements of birds
so that breeding can only occur in a place and time of adequate proximal supplies
of food and water.
111