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Irruptions, altitudinal and moult migrants
Waxwings cross the sea from Scandinavia to find more food
Irruptions
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.
Altitudinal migrants
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.
Moult migrants
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.
Last modified: 11 November 2005