Original by Dr Margaret Rowe, 2002
Last revised by the Department of the Environment and Heritage, December 2004
ISBN 0 6425 4820 X
Long distance migration of shorebirds
Many shorebirds migrate, making long-distance journeys, spanning the globe in a north-south direction. Most of the long-distance migrating birds breed in the Arctic regions, during the Arctic summer. Some 2 million of these birds—many weighing as little as 30 grams—make a yearly trip of 25,000 kilometres to Australia and back. During a 20 year lifetime, a long distance migrating shorebird would travel over 400,000 km.
These migrations are tuned to the seasons and are a response to the fact that when it is winter in the northern hemisphere it is summer in the southern hemisphere.
They arrive in Australia in August or September and leave in March or April, to return to their breeding grounds in the tundra areas of Siberia and, for some, Alaska, to breed in June and July.
While in Australia, they live along quiet stretches of the coast or inland waters, spreading out over the wet sand or mud.They feed on small worms and a variety of molluscs and crustaceans.On the coast, as the tide comes in and covers the feeding areas, the birds move onto the remaining patches of dry sand, roosting there in flocks until the tide recedes.
Because of the spherical shape of the earth, the sun's rays strike the earth's surface more directly at the equator and at much lower angles nearer to the poles. This has the effect of spreading the sun's rays out over a larger area near the poles. In addition, near the poles, the sun's rays have to pass through more atmosphere before striking the ground. As a result, the climate is generally a lot colder near the poles than near the equator.
Figure 12 Why it is colder near the poles than near the equator
As well as spinning on its own axis, the earth revolves on a yearly path around the sun. Because the earth's axis is tilted, the sun's rays strike parts of the earth at different angles at different times of year. During the northern summer, countries near the North Pole are tilted a little more directly into the sun's rays. This creates summer in the northern hemisphere. At the same time, countries nearer the South Pole are tilted so that the sun's rays strike at a very low angle. This creates winter in the southern hemisphere.
During summer in the Arctic regions, daylight hours are long, almost 24 hours, and temperatures are warm enough to melt the snow. As the Arctic regions are gradually tilted further away from the sun, summer passes. Meanwhile, the southern hemisphere is tilted further into the sun and experiences, spring, summer and autumn.
This seasonal effect is not nearly so great near the equator as it is towards the North and South Poles. Nearer to the poles, the change in the angles of the sun's rays during the year is greater, and the influence on the weather and on the number of hours of daylight each day is greater.
Figure 13 The seasons–why do we have summer, autumn, winter and spring?
Each year over 2 million migratory shorebirds travel between their breeding grounds and their wintering grounds in Australia and New Zealand. They make the journey, stopping at a few places on the way for two or three weeks to build up their reserves of fat for the next stage of their journey. The places where they pause to feed are called stopovers or staging sites.
The routes they travel are called flyways. On a map of the world, a few main flyways can be drawn.Australia is in the East Asian-Australasian Flyway.This flyway extends from the Arctic Circle through eastern and south-eastern Asia to Australia and New Zealand. Flyways provide chains of wetlands, usually coastal mudflats, which provide abundant food.These well stocked "snack bars" along the journey are essential to the survival of the birds. The birds cannot afford delays or food shortages if they are to migrate and breed successfully. These wetlands are very important.
These web pages on the ABC site show pictures and tell the story of migration:
" Wader Birds off to Siberia" http://www.abc.net.au/science/scribblygum/March2000/default.htm
- The care of the wetlands along the flyways, and the future existence of the birds that depend on them, requires international cooperation. Why?
Figure 14 A map of the East Asian-Australasian Flyway
Birds need a year-round food supply as well as suitable nesting sites. It is believed that this may explain why many shorebirds migrate.
The short summer in the Arctic tundra provides abundant nesting sites and a rich supply of food. The low vegetation of ground-hugging shrubs, mosses and lichens is dotted with pools of water and the area swarms with insects. These provide plenty of food for the newly hatched chicks of shorebirds.
The birds spread over millions of square kilometres, each pair defending their own territory. Eggs are laid and chicks grow rapidly, feeding on nutritious "insect soup". This convenient supply of fast food, ideal for raising chicks, is not available in other parts of the world. On the other hand, the birds could not survive the freezing weather of the Arctic tundra regions during the northern autumn, winter or spring.
The Arctic tundra supports an incredible quantity of life with its dense populations of lemmings, Arctic wolves, hares, owls, ptarmigan and other birds. Some of these feed on the eggs and chicks of shorebirds. It is believed that in years when the lemming population is low, shorebirds are more likely to fall prey to wolves and owls.
The adult birds spend no more than eight weeks in the tundra, flying south again, leaving their chicks to follow a little later. They travel, in stages, over several weeks, to more temperate regions in the southern hemisphere where they spend spring, summer and early autumn on coastal or inland wetlands. These sites are known as the wintering grounds because the birds are avoiding winter in the country where they hatched. Here they feed on small creatures found in the tidal sand- or mud-flats or shallow fresh water. Food includes shellfish, worms and insects.
Figure 15 How birds spend their year
Young birds, juveniles, do not return home to the Arctic the following winter, and do not breed; they remain in southern hemisphere in the wintering grounds for at least one Arctic summer, sometimes more.
In summary, the adult birds spend approximately two months in their Arctic breeding grounds in the northern hemisphere, five or six months in their wintering grounds in the southern hemisphere and about two months on each journey.
Long-distance migration involves flights of 4000 km and more—sometimes up to 8000 km—nonstop. Flocks of these shorebirds fly by day and night, over land and sea at altitudes of around 3000 to 8000 metres. They can adjust their altitude to avoid strong winds and take advantage of tail winds.They fly at speeds of approximately 30 to 60 kph.
Birds have an inbuilt body clock which tells them when it is time to migrate. It is sensitive to hormones that are made in the bird, in response to changes in the number of daylight hours each day as the seasons change.
The exact time of leaving may depend on the weather. Migratory shorebirds are sensitive to the weather patterns. Large flocks leave for long flights when weather and winds are in their favour. Some birds seem to have an inbuilt barometer.This allows them to detect changes in air pressure.They can detect an approaching storm, set off on their journeys during extensive high-pressure systems, and choose the best altitudes for long flights.
Before it is time to fatten up for migrations, birds complete their moult, replacing the important flight feathers. Moulting is gradual— the bird is able to fly at all stages.
Over two or three weeks in late summer the shorebirds put on weight, storing a layer of fat under their skin and developing larger flight muscles. The fat will be used as fuel during the journey. Birds gain about one third or more of their usual body weight. Some almost double their weight. Some organs in the birds' bodies, not essential for flight, may shrink, reducing unnecessary weight.
Birds use their stored fat as fuel and also need to stop a few times during their long journey. They renew their fat layer by feeding for two or three weeks. The birds rely on stopover sites to provide plentiful food—"fast food" which is vital for their journey. In some conditions, such as bad weather or poor food supply, birds turn back, returning to their wintering grounds. The birds that reach the breeding grounds must be well fed and in good condition so that they can breed successfully. If humans destroy or damage these feeding grounds, the stopovers, the birds will have no future.