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Migration
Migration

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1.1 Examples of animal migration

The humpback whale travels further than any other mammal (Video 2).

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Transcript: Video 2 The humpback whale (Megaptera novaeangliae).

NARRATOR:
A female humpback whale and her calf. Every few years, she will travel 3000 miles from the rich waters of the Antarctic to these warm, but comparatively sterile, waters of the Pacific, to give birth to a single calf.
End transcript: Video 2 The humpback whale (Megaptera novaeangliae).
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Video 2 The humpback whale (Megaptera novaeangliae).
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Travelling long distances over land may mean coping with a changing terrain. The migratory route of the bar-headed goose (Anser indicus) takes it over the peaks of the Himalayas at an altitude of around 9000 m (Video 3). The altitude record for a bird is 11 227 m, held by a griffon vulture.

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Transcript: Video 3 The bar-headed goose.

NARRATOR:
The bar-headed goose - it winters on the plains of Northern India. And each spring it cuts through the Himalayan barrier, threading its way through high glaciated valleys to reach its nesting grounds.

[MUSIC PLAYING]

These geese hold what is surely the altitude record for migrating birds. Many years ago, an astronomer in Northern India was photographing the Moon. When he developed the prints, one showed a skein of geese silhouetted against the Moon's disc. From their size, he calculated that they must have been flying at 29,000 feet. That's almost the height of Everest.
Bar-heads breed in what must be some of the most remote and desolate places on Earth. Most nest on soda lakes on the Tibetan Plateau. But a few travel on north and west to the Tian Shan and Pamir. Some 200 pairs nest on Karakum. Another 50 or so come here, to Lake Rankoo, a shallow salt lake hard by the border with Xianjiang.

[GEESE HONKING]

Almost no rain falls here. And even in winter, there's little snow. The lake's surroundings are a high, dry desert.
The geese arrive in late May, when the lake is still frozen. They settle on barren salty islands. The female digs a hole in the caustic soil, and lines it with down from her breast.
In this simple nest, she lays between two and five eggs. It will take them nearly a month to hatch. She shoulders the burden of all the incubating. But while she sits, her mate stands guard.
Though nesting on islands keeps wolves and foxes at bay, there are other dangers - ravens. When the first clutches hatch, a few addled eggs are left in the abandoned nests. These attract scavengers from the surrounding mountains. But given the chance, ravens take eggs from any unguarded nest.
Most eggs do hatch safely. The chicks remain in the nest for a day. Then their mother calls them down to the water.

[HONKING AND CHEEPING]

Rankoo is very shallow. Much of the lake is only three to six feet deep. In summer, its corrosive waters warm rapidly. And there are prolific hatches of midges, full of protein for the growing goslings.
End transcript: Video 3 The bar-headed goose.
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Video 3 The bar-headed goose.
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Even with a long distance to cover, the arctic tern does not take a direct route and also makes a stop in the North Atlantic to feed on plankton (Figure 3).

Described image
Figure 3 Migration route of the arctic tern. After setting out from their breeding sites in Greenland and Iceland (yellow line) the birds feed in the North Atlantic (red circle) and then follow one of two southward migration routes to reach their overwintering sites in the Antarctic; they then return (orange line) to their breeding sites, a round trip of 70 900 km.
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This figure is a simplified world map on which the migration routes of the artic tern are shown as arrowed lines. The birds travel a relatively short distance from their breeding sites in Greenland and Iceland to an area in the North Atlantic, where they make a fuel stop. Then they head south over the Atlantic, passing close to the northwest coast of Africa, after which the route branches. One branch heads southwest, towards the northeast coast of South America; it follows the coastline, then turns southeastwards to terminate in the wintering grounds of the Antarctic. The other southward migration route follows the west coast of Africa down to the tip of the continent and then turns southwest and on to the Antarctic wintering grounds. There is a single return route, which snakes over the Pacific and Atlantic oceans, heading first north, then northwest up to the northeast of South America, then north again up to the fuel stop in Greenland and Iceland, and finally back to the North Atlantic breeding areas.

Figure 3 Migration route of the arctic tern. After setting out from their breeding sites in Greenland and Iceland (yellow line) the birds...

One of the most amazing of all bird migrations is undertaken by the bar-tailed godwit (Limosa lapponica), a wading bird, frequently found in marshes or mudflats where it forages for food (Figure 4).

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Figure 4 Bar-tailed godwits feeding and taking off.
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This figure shows a flock of bar-tailed godwits feeding and taking off from a strip of land near water. They are medium-sized birds with a winter plumage that is mainly grey-brown. There are black markings on the wings and barring on the tail.

Figure 4 Bar-tailed godwits feeding and taking off.

It has been possible to tag individuals with satellite transmitters that encode position data on a radio signal, a process called telemetry. The data obtained from one bird showed that it had travelled 11 680 km (7258 miles), non-stop, over nine days to reach New Zealand from the Alaskan breeding grounds (Figure 6). This bird (E7) flew at an average speed of 56 km per hour (34.8 miles per hour) at an altitude of 3000-4000 m. Along the way, she lost more than half her body mass. The reasons for this extraordinary journey are the same for the majority of bird and other animal migrations - to increase both survival and reproductive success.

Described image
Figure 5 Flight paths and timings of the bar-tailed godwit E7. The journey from Alaska to New Zealand is non-stop. The return journey is achieved in two stages. A, Yukon Delta, Alaska, 5 May to 30 August; B, Miranda, New Zealand, 7 September to 17 March; C, Yalu Jiang, China, 24 March to 1 May.
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This figure is a simplified world map on which the flight paths of a single female bar-tailed godwit between sites in Alaska (marked A), New Zealand (B) and China (C) are shown as white arrowed lines. White circles at points along these lines indicate the locations at which satellite signals, 36-40 hours apart, were received. From start to finish of the direct outward journey from Alaska to New Zealand, there are five signal reception sites, which are fairly uniformly spaced, indicating a fairly constant flight speed. The five signal reception sites for the first stage of the return journey, from New Zealand to China, get progressively closer together, indicating a reduction in flight speed as the journey progressed; the last two are very close together, indicating that the final miles of the approach to Yalu Jiang were very slow. For the second stage of this journey, from China to Alaska, the one satellite signal between the start and finish sites was received after about 60 per cent of the journey was completed, indicating some slowing down.

Figure 5 Flight paths and timings of the bar-tailed godwit E7. The journey from Alaska to New Zealand is non-stop. The return journey is ...

For some species, there may be no opportunity to learn a route prior to setting off. Young ospreys born on Martha's Vineyard, an island off the east coast of Massachusetts, do not migrate with their parents but set off alone to spend the winter in Central or South America (Video 4). They are thought to find their way by flying southwards, keeping over land. It may be 18 months before they make the return trip.

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Transcript: Video 4 The ospreys of Martha's Vineyard.

[MUSIC PLAYING]

NARRATOR:
Ospreys are not only masters of the air ... but of the sea as well.

[MUSIC PLAYING]

Every year, they take part in an extraordinary migration of around 6000 miles between the perfect nesting sites in the north and the best fishing - way, way south. Our journey begins north of New York, in Martha's Vineyard. Sixty-five pairs of ospreys breed on this little island - and they're all about to leave.
Osprey biologist Rob Bierregaard has been studying their migration for the last five years. He attaches little solar-powered transmitters to wild ospreys. Via satellites, they beam the birds' location back to him, so he can see where they are, anywhere on the planet.
End transcript: Video 4 The ospreys of Martha's Vineyard.
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Video 4 The ospreys of Martha's Vineyard.
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