The discovery, by Ben Sheldon, Professor of Field Ornithology at Oxford University, runs contrary to existing ideas about evolution.

Since the days of Charles Darwin, it has been thought that differences established themselves among members of the same species only if they were totally separated from each other, such as the finches on the Galápagos islands.

It has always been supposed that if animals who acquired some slight new difference then mixed with other members of their species, they would interbreed and the difference would quickly vanish again.

Professor Sheldon and his team of birdwatchers from the Edward Grey Institute of Field Ornithology in Oxford have shown that this is not true.

They have been weighing great tit nestlings in the wood for many years and it has emerged that they are bigger every year in the area called Wytham Great Wood. The parents of the nestlings have also been trapped, and have also been found to be bigger.

By contrast, in the area of Wytham called Marley Wood, 1¼m away, parents and offspring have been smaller.

Professor Sheldon, standing in Wytham Wood among hundreds of nest boxes that his team have put up, said that the difference appeared to be connected to the number of breeding sites available in the two areas.

The team in Wytham Great Wood has put up relatively few nest boxes. Fewer nests in an area means that there is more food available in the breeding season and the birds like to nest there. So the heavier great tits, who are bullies, drive away the smaller ones. The heavy birds have heavier youngsters and so a population of heavy birds has grown up in this area.

Meanwhile, the smaller, weaker birds go to nest in Marley Wood, where there are plenty of nest boxes on the trees. Here, the large number of families occupying them means less food and a smaller, weaker population has emerged. It makes no difference that the two populations nest so near to each other, or that they mix happily in winter.

The Wytham Wood research is published today in the journal Nature.

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