partly digested food , are subjected to an additional drain on their supplies of body
water and salts.
Birds often migrate long distances to breed in areas where the climate is more
equitable, and supplies of food and water are more abundant. The process of migration would appear to place a considerable burden on the water metabolism of
birds . YAPP (1956) indeed, considers water loss as the single greatest limiting factor
in bird migration. TUCKER (1968) has conservatively calculated that a small bird,
the budgerygah, can fly at least 500 km before becoming seriously dehydrated, and
it seems likely that larger birds could travel considerably further than this . The
ability of some birds to make long transoceanic flights is, nevertheless, quite a remarkable osmoregulatory feat.
The role of physiological factors in initiating the migration of birds has been
subject to considerable speculation. The possible roles of the endocrine system have
received their share of this attention which is reflected in the following quotation:
'There is little doubt that predisposition to migration is a hormone induced state
probably a complex of several hormones' (HOHN, 1961). Physiological changes
that occur prior to migration may include, an activation of the reproductive system,
the laying down of reserves of fat and the onset of restless nocturnal activity called
Zugunruhe.Such changes all could involve hormonal processes but not necessarily as
an initiating influence. FARNER (1955) does not consider that gonadal sex hormones
playa direct role in avian migration, especially as sexually immature and castrate
birds also migrate and injections of sex hormones do not mimic the associated preparatory metabolic changes. MARSHALL (1961), however, felt that, with the available evidence, the gonads could not be excluded unequivocally. Alterations in
thyroid function have also been considered (see FARNER, 1955); the Zugunruhe
is reminiscent of a hyperthyroid condition but this would not be consistent with
the laying down of fat reserves . Migration, and its physiological preparation, is
undoubtedly a complex process involving the interaction of many factors . Changes
in endocrine function must inevitably occur, but just how directly these are involved is not at present clear.
7. Osmoregulation in Two Desert Birds; the Budgerygah
and the Zebra Finch
The budgerygah, Melopsittacus undulatus, is a small parrot weighing about 30 g
and the zebra finch, Taeniopygia costanotis, is a passerine species of about 12 g.
Both these species eat seeds and live in the hot arid interior regions of Australia.
These areas are shared with marsupials, some of which were discussed in the last
chapter. The climate is characterized by a low and unpredictable rainfall combined
with high temperatures and low humidities. There are seasonal droughts when little
or no rain falls and these sometimes may last for three or four years .
DONALD FARNER and DOMINIC SERVENTY and their collaborators (OKSCHE et
al., 1963) reported that zebra finches kept outdoors in captivity in Perth, Western
Australia, could survive for extended periods with little or no water to drink. CADE,
et aI., (1965) found that 8 out of 16 domesticated zebra finches placed on a diet
of seeds with no water to drink, survived for more than 90 days when kept in the
130
water and salts.
Birds often migrate long distances to breed in areas where the climate is more
equitable, and supplies of food and water are more abundant. The process of migration would appear to place a considerable burden on the water metabolism of
birds . YAPP (1956) indeed, considers water loss as the single greatest limiting factor
in bird migration. TUCKER (1968) has conservatively calculated that a small bird,
the budgerygah, can fly at least 500 km before becoming seriously dehydrated, and
it seems likely that larger birds could travel considerably further than this . The
ability of some birds to make long transoceanic flights is, nevertheless, quite a remarkable osmoregulatory feat.
The role of physiological factors in initiating the migration of birds has been
subject to considerable speculation. The possible roles of the endocrine system have
received their share of this attention which is reflected in the following quotation:
'There is little doubt that predisposition to migration is a hormone induced state
probably a complex of several hormones' (HOHN, 1961). Physiological changes
that occur prior to migration may include, an activation of the reproductive system,
the laying down of reserves of fat and the onset of restless nocturnal activity called
Zugunruhe.Such changes all could involve hormonal processes but not necessarily as
an initiating influence. FARNER (1955) does not consider that gonadal sex hormones
playa direct role in avian migration, especially as sexually immature and castrate
birds also migrate and injections of sex hormones do not mimic the associated preparatory metabolic changes. MARSHALL (1961), however, felt that, with the available evidence, the gonads could not be excluded unequivocally. Alterations in
thyroid function have also been considered (see FARNER, 1955); the Zugunruhe
is reminiscent of a hyperthyroid condition but this would not be consistent with
the laying down of fat reserves . Migration, and its physiological preparation, is
undoubtedly a complex process involving the interaction of many factors . Changes
in endocrine function must inevitably occur, but just how directly these are involved is not at present clear.
7. Osmoregulation in Two Desert Birds; the Budgerygah
and the Zebra Finch
The budgerygah, Melopsittacus undulatus, is a small parrot weighing about 30 g
and the zebra finch, Taeniopygia costanotis, is a passerine species of about 12 g.
Both these species eat seeds and live in the hot arid interior regions of Australia.
These areas are shared with marsupials, some of which were discussed in the last
chapter. The climate is characterized by a low and unpredictable rainfall combined
with high temperatures and low humidities. There are seasonal droughts when little
or no rain falls and these sometimes may last for three or four years .
DONALD FARNER and DOMINIC SERVENTY and their collaborators (OKSCHE et
al., 1963) reported that zebra finches kept outdoors in captivity in Perth, Western
Australia, could survive for extended periods with little or no water to drink. CADE,
et aI., (1965) found that 8 out of 16 domesticated zebra finches placed on a diet
of seeds with no water to drink, survived for more than 90 days when kept in the
130