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Australia Tourism » Adaptation Migratory Iifestyle
Adaptation Migratory Iifestyle, Australia
ADAPTATIONS FOR A MIGRATORY LIFESTYLE
Migratory shorebirds have an energetically expensive lifestyle. On top of the costs associated with annual breeding and moult that all birds have to cope with, they devote three to four months a year to migration. There is more to migration than extraordinarily long direct flights: each flight is preceded by a long period of preparatory feeding, as birds accumulate the fuel they will need for a non-stop flight of several days.
FEEDING
A successful migratory, shorebird needs to be a superb foraging machine. With a wide variety of morphological adaptations, different species of shorebird are capable of living in habitats as diverse as tropical wetlands and cold temperate shorelines. For example, Grey Plovers are adept at scanning the mudflats in search of the tiny rear-end of a defecating bristle-worm, then dashing over and grabbing the worm before it can retreat into the mud. Red Knots hunt buried bivalves below the mud surface, using receptors in the bill-tip to detect pressure differentials in the mud with such sensitivity that they can locate a hard buried object at least 5 cm away. Most of the fuel used in migration is fat - it releases more energy per gram than any other form of tissue. However, protein is also important to the fuelling of migration. In the early stages of weight-gain, shorebirds increase the size of their digestive oroans, thereby increasing their ability to turnpoint in having a muscular stomach (there is no food to digest during the flight and the extra weight costs energy). On the other hand, there is an advantage to increasing the size of flight muscles, especially as initially they have to carry a heavy load of fat. Shorebirds have a remarkable ability to undergo rapid changes in organ size, and just before they depart on migration, they reallocate protein from the digestive organs to the flight muscles.
BREEDING
Shorebirds moult into breeding plumage before they reach the nesting areas; these offer marvellous camouflage on the tundra and are sufficiently warm to keep birds insulated in subzero temperatures.
Shorebirds have also evolved the most diverse range of mating behaviours seen in any group of birds. Examples include monogamous species highly faithful to both their mates and their nesting sites; lekking species, in which the males mate with as many females as possible and never see their offspring; and sequentially polyandrous species in which the female leaves a male with one clutch of eggs, then immediately lays another clutch and brings that one up herself.
By the time the chicks hatch, the Arctic has bloomed, with enormous amounts of food to support the growing birds. Chicks hatch in downy plumage, and within minutes of hatching are capable of walking and feeding themselves. They are, nevertheless, completely dependent on their parents for a period. While the chicks' down offers wonderful camouflage, it doesn't offer as good insulation as adult plumage. This is a problem, especially in very small chicks, which lose heat rapidly. They get around it with a behaviour called brooding. Adults have a pair of brood patches - areas of bare, highly vascular skin on each side of the belly. Chicks have large blood vessels at the back of their necks. When the chicks are getting cold, the parent calls them in to press the back of their necks against the adult brood patches. This raises the body temperature of the chicks with extraordinary speed.
As soon as the chicks are large enough to fly and survive without brooding, the adults abandon them and set off on southwards migration. They have a tight schedule to meet, because if they do not migrate to their non-breeding areas and complete their annual flight feather moult on time, they will not be able to attempt breeding in the next summer.
The chicks remain a little longer, fattening up before the Arctic freezes over. Like the adults, they migrate south using a range of navigational cues - the stars, the sun and the Earth's magnetic field help set them in the right direction. Unlike adults, however, they lack experience: it takes the young birds longer to find food on the staging areas and they typically arrive in non-breeding areas such as Australia a month or two after the adults.
This may be in part why the young of most species of long-distance migrants take a long time to mature - they are not ready to migrate north with the adults for the following breeding season. Instead, they remain in the non-breeding areas learning to feed in coastal habitats, and finding the non-breeding area to which they will probably remain faithful for the rest of their life. After two years - or three, or four, depending on the species - they are ready to join the adults in their migrations to the other end of the world.
Migratory shorebirds have an energetically expensive lifestyle. On top of the costs associated with annual breeding and moult that all birds have to cope with, they devote three to four months a year to migration. There is more to migration than extraordinarily long direct flights: each flight is preceded by a long period of preparatory feeding, as birds accumulate the fuel they will need for a non-stop flight of several days.
FEEDING
A successful migratory, shorebird needs to be a superb foraging machine. With a wide variety of morphological adaptations, different species of shorebird are capable of living in habitats as diverse as tropical wetlands and cold temperate shorelines. For example, Grey Plovers are adept at scanning the mudflats in search of the tiny rear-end of a defecating bristle-worm, then dashing over and grabbing the worm before it can retreat into the mud. Red Knots hunt buried bivalves below the mud surface, using receptors in the bill-tip to detect pressure differentials in the mud with such sensitivity that they can locate a hard buried object at least 5 cm away. Most of the fuel used in migration is fat - it releases more energy per gram than any other form of tissue. However, protein is also important to the fuelling of migration. In the early stages of weight-gain, shorebirds increase the size of their digestive oroans, thereby increasing their ability to turnpoint in having a muscular stomach (there is no food to digest during the flight and the extra weight costs energy). On the other hand, there is an advantage to increasing the size of flight muscles, especially as initially they have to carry a heavy load of fat. Shorebirds have a remarkable ability to undergo rapid changes in organ size, and just before they depart on migration, they reallocate protein from the digestive organs to the flight muscles.
BREEDING
Shorebirds moult into breeding plumage before they reach the nesting areas; these offer marvellous camouflage on the tundra and are sufficiently warm to keep birds insulated in subzero temperatures.
Shorebirds have also evolved the most diverse range of mating behaviours seen in any group of birds. Examples include monogamous species highly faithful to both their mates and their nesting sites; lekking species, in which the males mate with as many females as possible and never see their offspring; and sequentially polyandrous species in which the female leaves a male with one clutch of eggs, then immediately lays another clutch and brings that one up herself.
By the time the chicks hatch, the Arctic has bloomed, with enormous amounts of food to support the growing birds. Chicks hatch in downy plumage, and within minutes of hatching are capable of walking and feeding themselves. They are, nevertheless, completely dependent on their parents for a period. While the chicks' down offers wonderful camouflage, it doesn't offer as good insulation as adult plumage. This is a problem, especially in very small chicks, which lose heat rapidly. They get around it with a behaviour called brooding. Adults have a pair of brood patches - areas of bare, highly vascular skin on each side of the belly. Chicks have large blood vessels at the back of their necks. When the chicks are getting cold, the parent calls them in to press the back of their necks against the adult brood patches. This raises the body temperature of the chicks with extraordinary speed.
As soon as the chicks are large enough to fly and survive without brooding, the adults abandon them and set off on southwards migration. They have a tight schedule to meet, because if they do not migrate to their non-breeding areas and complete their annual flight feather moult on time, they will not be able to attempt breeding in the next summer.
The chicks remain a little longer, fattening up before the Arctic freezes over. Like the adults, they migrate south using a range of navigational cues - the stars, the sun and the Earth's magnetic field help set them in the right direction. Unlike adults, however, they lack experience: it takes the young birds longer to find food on the staging areas and they typically arrive in non-breeding areas such as Australia a month or two after the adults.
This may be in part why the young of most species of long-distance migrants take a long time to mature - they are not ready to migrate north with the adults for the following breeding season. Instead, they remain in the non-breeding areas learning to feed in coastal habitats, and finding the non-breeding area to which they will probably remain faithful for the rest of their life. After two years - or three, or four, depending on the species - they are ready to join the adults in their migrations to the other end of the world.
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