More blackcaps from Germany and native British chiffchaffs are giving up on the 4,000-mile journey to sub-Saharan Africa, hoping to tough out the British winter in exchange for first pick of the best breeding grounds in the spring, British Trust for Ornithology spokesman Graham Appleton told AFP.

The migratory change is an example of evolution in action, he explained.

Some blackcaps with a genetic defect causing them to fly west instead of south for the winter have presumably been flying to Britain and perishing in the cold winters for some time, experts said.

As British winters grow warmer, and birdwatchers are more diligent about providing food for the birds, more such blackcaps and stay-at-home chiffchaffs are surviving in Britain.

In fact, since they are better able to tell when winter is over, the blackcaps can return to Germany first to snatch the best breeding grounds and the chiffchaffs can do likewise in Britain.

They can then produce more offspring, giving them a genetic advantage -- and in turn causing more to winter in Britain.

So, are these birds an example of a new lazy breed?

"These birds are adopting a strategy which means they have to fly less and so you could call that a lazy strategy," Appleton admitted.

"But it seems to be working for them," he added.

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