approaches more closely that of the environment. Physiologically the rates of heat
loss by radiation, conduction and convection can be altered by changes in cutaneous blood flow, modifying posture such as by spreading out the wings, extending the foot swim web, and fluffing or flattening the plumage. Such physical heat
loss may playa predominant role in the ability of the bird to maintain its body
temperature in hot situations. It can be supplemented, to various degrees, by facilitation of the evaporation from the respiratory tract. The temperature at which
such processes are initiated varies in different species (KING and FARNER, 1964)
depending on the normal body temperature, metabolic rate, the surface area and
the birds insulation. A high body temperature could favour life in hot desert
regions, as this would facilitate heat loss to the environment, but a comparison of
the body temperatures of birds indicates there are no such differences of adaptive
significance to a desert life.
At high ambient temperatures increased evaporative water loss inevitably occurs, and this is brought about physiologically by an increase in the ventilation of
the lungs; initially with a change in tidal volume and then, at about 42
0
, by panting
which results from increased respiratory movements, and in many birds by an accelerated movement of the hyoid apparatus called 'gular fluttering' (BARTHOLOMEW,
LASIEWSKI, and CRAWFORD, 1968). At an environmental temperature of 40
0 such
evaporative water loss may account for one to two-thirds of the heat dissipation.
Flying results in a considerable increase in metabolic rate and accompanying increases in evaporative water loss. These processes have been carefully measured by
VANCE TUCKER (1968) using budgerygahs flying in a wind tunnel. Evaporative water loss and oxygen consumption at 20
0 are 5 times as great in flight as at rest. When
the ambient temperature was increased during flying, the oxygen consumption
stayed the same, but evaporative water loss increased and was three times as great
at 36
0 as at 20
0
•
Evaporative water loss in birds can be altered somewhat in different physiological circumstances. Birds, like mammals, withstand elevated body temperatures only to a very limited extent and cannot survive a body temperature of more
than about 45
0 for any period of time. Toleration of hyperthermia, which results
from storage of heat, with a reduction of evaporative water loss, can occur but is
limited in its range . This can be extended by adopting a pattern of nocturnal hypothermia, followed by diurnal hyperthermia (see KING and FARNER, 1964), but the
importance of such heat storage in water conservation is not clear. When birds are
deprived of water, like mammals, they may reduce evaporative water loss and as
a result undergo an increased body temperature. This has been observed in desert
birds like the ostrich (CRAWFORD and SCHMIDT-NIELSEN, 1967), the budgerygah
(GREENWALD et al., 1967) and the zebra finch (CALDER, 1964). The physiological
causes of such adjustments in water loss are uncertain, but if the metabolic rate
changes, as in nocturnal hypothermia, the thyroid gland could be involved. The
thyroid function in such birds does not, however, appear to have been investigated
from this aspect.
Birds living in arid areas adopt patterns of behaviour that reduce evaporative
water loss . Such adaptations in the mammals include avoiding the heat of the day
by seeking refuge in cooler places, such as burrows, and coming out to feed during
the cool night hours. Few birds are, however, nocturnal and even fewer are fossorial
114
loss by radiation, conduction and convection can be altered by changes in cutaneous blood flow, modifying posture such as by spreading out the wings, extending the foot swim web, and fluffing or flattening the plumage. Such physical heat
loss may playa predominant role in the ability of the bird to maintain its body
temperature in hot situations. It can be supplemented, to various degrees, by facilitation of the evaporation from the respiratory tract. The temperature at which
such processes are initiated varies in different species (KING and FARNER, 1964)
depending on the normal body temperature, metabolic rate, the surface area and
the birds insulation. A high body temperature could favour life in hot desert
regions, as this would facilitate heat loss to the environment, but a comparison of
the body temperatures of birds indicates there are no such differences of adaptive
significance to a desert life.
At high ambient temperatures increased evaporative water loss inevitably occurs, and this is brought about physiologically by an increase in the ventilation of
the lungs; initially with a change in tidal volume and then, at about 42
0
, by panting
which results from increased respiratory movements, and in many birds by an accelerated movement of the hyoid apparatus called 'gular fluttering' (BARTHOLOMEW,
LASIEWSKI, and CRAWFORD, 1968). At an environmental temperature of 40
0 such
evaporative water loss may account for one to two-thirds of the heat dissipation.
Flying results in a considerable increase in metabolic rate and accompanying increases in evaporative water loss. These processes have been carefully measured by
VANCE TUCKER (1968) using budgerygahs flying in a wind tunnel. Evaporative water loss and oxygen consumption at 20
0 are 5 times as great in flight as at rest. When
the ambient temperature was increased during flying, the oxygen consumption
stayed the same, but evaporative water loss increased and was three times as great
at 36
0 as at 20
0
•
Evaporative water loss in birds can be altered somewhat in different physiological circumstances. Birds, like mammals, withstand elevated body temperatures only to a very limited extent and cannot survive a body temperature of more
than about 45
0 for any period of time. Toleration of hyperthermia, which results
from storage of heat, with a reduction of evaporative water loss, can occur but is
limited in its range . This can be extended by adopting a pattern of nocturnal hypothermia, followed by diurnal hyperthermia (see KING and FARNER, 1964), but the
importance of such heat storage in water conservation is not clear. When birds are
deprived of water, like mammals, they may reduce evaporative water loss and as
a result undergo an increased body temperature. This has been observed in desert
birds like the ostrich (CRAWFORD and SCHMIDT-NIELSEN, 1967), the budgerygah
(GREENWALD et al., 1967) and the zebra finch (CALDER, 1964). The physiological
causes of such adjustments in water loss are uncertain, but if the metabolic rate
changes, as in nocturnal hypothermia, the thyroid gland could be involved. The
thyroid function in such birds does not, however, appear to have been investigated
from this aspect.
Birds living in arid areas adopt patterns of behaviour that reduce evaporative
water loss . Such adaptations in the mammals include avoiding the heat of the day
by seeking refuge in cooler places, such as burrows, and coming out to feed during
the cool night hours. Few birds are, however, nocturnal and even fewer are fossorial
114