The Friends of Charles Darwin

Great tits are evolving in different ways less than 1½ miles from each other in the same wood. In one area of Wytham Wood, just outside Oxford, they are getting larger, while in another part they are getting smaller. 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…

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.