Balmy winter puts chill on bird migration

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OSLO, Norway (Reuters) -- Some European birds have failed to fly south for the winter, apparently lured to stay by weeks of mild weather that experts widely link to global warming.

Birds including robins, thrushes and ducks that would normally fly south from Scandinavia, for instance, have been seen in December -- long after snow usually drives them south. And Siberian swans have been late reaching western Europe.

"With increasing warmth in winter we suspect that some types of birds won't bother to migrate at all," said Grahame Madge, spokesman of the British Royal Society for the Protection of Birds (RSPB).

Many individual birds were leaving later, and flying less far.

One Swiss study this month suggested that Europe has just had the warmest autumn in 500 years. Frosts have crept south in the past week -- chilling any birds gambling that the entire winter will be balmy.

Madge said that Bewick's swans, for instance, which usually arrive in Britain in October from Siberia in Russia had apparently stopped for longer than usual in countries such as Estonia or the Netherlands because of plentiful food.

Birds cutting down on migration save vast amounts of energy on dangerous flights -- such as from the Arctic to Africa and back -- and can have the pick of northern breeding sites in spring. But they risk being killed by a snap cold spell.

"Some birds are much more common in winter here than they were about 30 years ago," said Geoffrey Acklam, a veteran amateur ornithologist who lives near Oslo where there is no snow but some overnight frosts.

"It's a result of a series of mild winters."

He said he first saw a robin in winter in the 1970s, but recently nine were spotted locally in one day. Migratory chiff chaffs, thrushes and field fares were also increasingly common.

The World Meteorological Organization said last week that 10 of the warmest years since records began in the 1850s were in the last 12 years -- 2006 ranks a provisional sixth.

Changing migratory patterns can also affect distant habitats. Hundreds of millions of birds fly from the Arctic as far as Australia and South America every year, where they can be food for other animals.

Experts say that the spring migration is becoming earlier.

"Birds are arriving earlier across Europe," said Endre Knudsen, a researcher at Oslo University. Birds often needed to race to grab the best nesting sites.

But he said it was risky because the arrival of migrants was sometimes out of step with the availability of insects and other food.

Almost all climate scientists reckon that human emissions of greenhouse gases from burning fossil fuels are driving up temperatures and could lead to more floods, erosion, desertification, spread of disease and rising sea levels.