Which birds migrate?
Not all birds migrate. A few, such as partridges, never move more than a kilometre or so from where they were born. These are called sedentary birds.
The most famous are long distance migrants, such as swallows, which breed in Europe and spend the winter in Africa. But you might be surprised to learn how many others are at it too. Even the blackbirds in your garden in January could well be winter visitors from Eastern Europe.
At least 4,000 species of bird are regular migrants. That’s about 40 per cent of the world’s total. But some parts of the world have a higher proportion of migrants than others.
In far northern regions, such as Canada or Scandinavia, most species migrate south to escape winter. In temperate regions, such as the UK, about half the species migrate – especially insect-eaters that can’t find enough food during winter.
In tropical regions, such as the Amazon rainforest, fewer species migrate, since the weather and food supply there are more reliable all year round. Different species migrate in different ways.
Irruptions, altitudinal and moult migrants
Irruption is a mass arrival of birds that do not usually visit the UK in large numbers. This happens with some northern species, such as waxwings, when their population grows too large for the food supply.
For example. once some waxwings have eaten all the berries in their usual Scandinavian winter quarters, they have to cross the sea to the UK to find more. Irruptions only happen every 10 years or so; we can't expect to see waxwings every winter.
Instead of migrating between north and south or east and west, some birds migrate up and down. This is called altitudinal migration - or vertical migration. Birds that breed in upland areas in summer head down to lowland areas in winter in search of a milder climate and more food.
Although the journey may not be long, it often involves quite a change in lifestyle. Altitudinal migrants in the UK include skylarks, meadow pipits and snow buntings.
Moulting is when birds shed their old feathers in order to grow a new set. All birds do this every year. But some, such as shelducks, lose all their flight feathers together and cannot fly for a while. This makes life quite risky, so shelducks migrate to do the job more safely.
In late summer, after breeding is over, they fly to the island of Heligoland in the North Sea - where they can moult with little disturbance or danger from predators. A few also fly to moulting sites closer to home, such as Bridgwater Bay in Somerset. They all return to their usual homes as soon as their new feathers have grown.
Summer, winter, passage and partial migrants
Summer visitors are birds that arrive in spring from the south to breed. Many are insect eaters. They spend summer here, then they – and their new young – return south in autumn.
They include swallows and martins, warblers, flycatchers, wheatears, whinchats, redstarts, nightingales, yellow wagtails, tree pipits, cuckoos, swifts, nightjars, turtle doves, hobbies, ospreys, terns and Manx shearwaters. Many other seabirds, such as puffins and gannets, also arrive on our shores in spring after spending the winter at sea.
Winter visitors are birds that arrive in autumn from the north and east to spend the winter in the UK, where the weather is milder and food is easier to find. In spring, they return to their breeding quarters.
They include fieldfares, redwings, bramblings, Bewick’s and whooper swans and many kinds of ducks, geese and wading birds. Many water birds also spend the winter on the sea around the UK coast, including common scoters, great northern divers and red-necked grebes.
Passage migrants are birds that stop off in the UK during their long journey north or south, such as green sandpipers and black terns. They use the UK like a service station, taking a few weeks during spring and autumn to refuel and rest before moving on.
Some species, such as dunlins, behave differently according to where they come from. The smaller dunlins that breed in Greenland and Iceland are passage migrants – stopping off with us on their way to west Africa. The larger dunlins that breed in Russia and northern Scandinavia stay with us for the whole winter.
Partial migrants are birds that migrate in some places, but not in others. For instance, most starlings that breed in the UK stay put for the winter. But starlings that breed in eastern Europe, where winter is much colder, migrate to the UK in winter. The same goes for chaffinches, robins, lapwings, coots and many other common birds.
Partial migration depends upon the weather, so it is never the same from one year to the next. Birds that hardly move at all in Britain the UK may migrate in huge numbers elsewhere. In Estonia, one birdwatcher counted 7,300 siskins, 6,200 great tits, 5,600 woodpigeons, 3,400 jays, 780 coal tits and 460 blue tits migrating in a single day!