Fly away Peter, come in Paul ... Britain's migratory birds flee the nest as global warming takes hold

Manmade climate change is likely to wreak havoc with migratory birds around the world and slash the populations of several species, a report warns today.

Birds that rely on British coastal habitats are particularly in danger, the World Wide Fund for Nature says. Rising sea levels will cover huge tracts of mudflat and shore marshes which in winter provide them with a crucial feeding ground.

The report says that the gradual global warming which appears to be under way may be the reason why several species are nesting earlier in the year in Britain. They includes species as diverse as the chaffinch, magpie, dipper and wren.

Furthermore, some birds whose British populations were at the northern limit of their range appear to be building up their numbers here as the climate becomes warmer. These include Cetti's warbler and two tropical and Mediterranean species which appear to be on verge of establishing UK breeding populations, the little egret and the Eurasian spoonbill.

The report says that half-a-dozen species which breed in tundra-like habitats in the coldest parts of Britain may stop nesting here if the climate warms substantially. These include the dotterel, ptarmigan and snow bunting, all mountain-top birds.

But the biggest impact of all could be on the millions of wildfowl and waders which fly south along Western Europe's coastline from their Arctic and sub-Arctic nesting grounds towards warmer climes for the winter. Their route is known as the eastern Atlantic flyway, and Britain's estuaries provide crucial feeding grounds along the way.

The marine life in the mudflats provide some birds with a winter-long food supply, while for others they are crucial refuelling stops during migratory flights which are thousands of miles long.

``Shore birds such as the sanderling, knot or dunlin are able to double their weight in fat after just a few days of frenzied feeding,'' the report says.

Apart from permanently covering the tidal mudflats with higher sea levels, global warming is projected to have another impact on the migratory birds. Migrations are often tied to ``resource flushes'' - times when food is particularly abundant. But if climate change shifts the lifecycles of the animals they feed on, the birds could find less food available when they arrive at stop-over points and breeding grounds.

The report points out that a sea-level rise of 1ft, which is projected for the end of the next century, will cover much of the mudflats. The Wash, probably the single most important shore-bird feeding ground in Europe, is likely to be one casualty.

The southern and eastern coasts of Britain are already sinking, irrespective of any rise in sea levels caused by global warming. Species assessed to be particularly at risk are the knot, grey and ringed plovers, bar-tailed godwit, shelduck and three species of goose - the barnacle, pink-footed and brent.

LOSSES

Knot - a small wading bird which nests in the Canadian High Arctic and Greenland and winters in British estuaries.

Snow bunting - nests around mountain peaks in Scotland, and spends the winter on the shoreline.

Ptarmigan - a grouse which lives on some of Scotland's highest mountain tops.

Whimbrel - a long-beaked wader which breeds in small numbers only in the extreme north of Scotland, and winters further south.

GAINS

Cetti's warbler - a small bird of marshes and riversides, it came here from the south in the 1960s. Over 200 breeding pairs.

Little egret - small pure-white herons from the Mediterranean, rare visitor to UK until the 1960s. May soon start to breed here.