Britain offers haven to Mediterranean bird

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The Independent Online
(First Edition)

BRITAIN has become a winter haven for a small bird that 30 years ago preferred to spend the coldest months of the year near the Mediterranean.

Scientists believe the regular winter appearance of the blackcap - a warbler rarely seen here in winter before 1960 - is a sign of either global warming, or the British love of the suburban bird table.

A study by German researchers, published in today's Nature, shows that some blackcaps have changed their migratory route dramatically in recent years which takes them up to 1,500 kilometres (930 miles) further north than their usual wintering grounds in Spain.

They suggest the change is due to 'improved wintering conditions' in Britain possibly brought about by the warmer winters recorded since the 1950s or the rise in the popularity of bird tables.

The study, by ornithologists at Heidelberg University, showed the change in the blackcap's migratory route is a genuine evolutionary development brought about by a change in the environment. They have shown that the blackcaps wintering in Britain are genetically programmed to migrate in a north-northwest direction from Germany, rather than the south-west route their ancestors took. Blackcaps that spend the summer breeding season here are genetically programmed to fly south to Spain in autumn.

'To our knowledge, this is the first case in any vertebrate in which a drastic and recent evolutionary change of behaviour has been documented and its genetic basis established,' they say.

In many birds migratory behaviour is inherited not learnt and the standard way of recording the migratory patterns of bird is to observe their movements in a circular cage at night during the migration period. The direction they tend to move corresponds well with the migration route they will follow.

William Sutherland, a biologist at the University of East Anglia, said the birds would have altered their migration routes in the past in response to climate changes during an ice age that lasted many centuries. The mystery is why the dramatic change observed by the researchers should have come about in the relatively short space of 30 years, he said.

'For such a rapid change to take place, the selection pressure must have been strong. One reason for the switch to the north-west route could be to take advantage of the increasing habit of providing food on bird tables.'

Another reason could be warmer winters. 'The need for such changes in migratory behaviour is likely to increase as a result of global warming,' he said.