Putting out seeds and bread for the birds in winter can do more than fatten them up — it can alter the course of their evolution, according to scientists.

They say that the British fondness for bird tables has caused a group of the European blackcap to break their custom of migrating to Spain and instead spend the winter season in Britain. In fewer than 30 generations, birds visiting Britain have evolved different-shaped wings and beaks.

The birds continued to breed side-by-side in the same southern German forests, but began to follow different winter migration routes after some discovered rich pickings and an increasingly mild winter climate in Britain.

Eventually, with different evolutionary pressures, they divided into two ecotypes — reproductively separate groups.

One group continued to fly south for the winter, migrating about 1,000 miles (1,600 km) to Spain to forage for olives and fruits. But about 10 per cent of the birds flew the 560 miles northwest to Britain where the colder weather was compensated for by a ready food supply from bird tables and by the shorter journey.

“The new northwest migratory route is shorter, and those birds feed on food provided by humans instead of fruits as the birds that migrate southwest do,” said Dr Martin Schaefer, from the University of Freiburg in Germany, who led the study.

As a consequence, birds migrating northwest have rounder wings, which provide better manoeuvrability but make them less suited for long-distance migration. They also have longer, narrower bills that are not so good for eating food such as olives during the winter. “It shows the profound impact of human activities on the evolutionary trajectories of species,” said Dr Schaefer.

If two ecotypes continue down their separate paths, they could ultimately become separate species. However, this would depend on how the habits of humans and local climates change over time. The findings, published in the journal Current Biology, challenge the conventional view that two populations have to be geographically separated to diverge into different species. Many scientists have argued that it would be unusual for gene pools to be kept separate for long enough for a new species to emerge.

“This is a nice example of the speed of evolution,” Dr Schaefer added. “It is something that we can see with our own eyes if we only look closely enough. It doesn't have to take millions of years.”

The blackcap (Sylvia atricapilla) is a greyish-coloured warbler, slightly smaller than a house sparrow, with a distinctive black or red-brown crown and forehead. During the breeding season the birds root around shrubs and trees for insects, spiders and caterpillars, but at other times they normally feed on fruit.

The birds were first spotted in Britain by bird ringers in the 1960s and the number of sightings has increased since then. The scientists have not established why the birds first started coming, but said that Britain could have been an occasional migratory route in the past that became more popular with an increase in bird tables. “We think it was when a lot more people started to use bird feeders after the Second World War,” said Dr Gregor Rolshausen, a co-author on the paper. “Before that the UK wasn’t that friendly to birds.”

Reported by Hannah Devlin