A male European blackcap gathers grass for its nest. (Michael Apel/Wikimedia Commons)

Putting out a bird feeder in the winter can have a dramatic effect on the evolution of migratory birds, researchers in Germany and Canada say.

In research published in Current Biology, scientists say a single population of European blackcaps has in a matter of decades split into groups that don't interbreed, despite living in the same forest for part of the year.

Bird researchers first noticed the divide in migratory patterns in 1959, after humans began offering the blackcaps food in the winter. One group migrates from Germany to the southwest, spending winters in Spain, while the other flies to the northwest to winter in Great Britain.

The two groups now show different adaptations based on the length of their migratory route and the food available to them in winter, the scientists said.

"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 Martin Schaefer of the University of Freiburg.

"As a consequence, birds migrating northwest have rounder wings, which provide better manoeuvrability but make them less suited for long-distance migration," said Schaefer.

The birds that spend their winters in the U.K. also have longer, narrower bills, which are less suited to eating fruit, such as olives, that make up 95 per cent of the diet of the birds wintering in Spain.

Blackcaps in the U.K. tend to feed on seeds and suet left in backyard feeders.

"[Our research] shows that we are influencing the fate not only of rare and endangered species, but also of the common ones that surround our daily lives," said Schaefer.

Schaefer said it isn't clear whether the two groups, or ecotypes, will ever separate to the point that they will become different species, but said the research is sparking debate over whether geographic separation is needed for one species to split into two.

"This is a nice example of the speed of evolution," he said. "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."

Keith Hobson, a bird conservation researcher with Environment Canada, also contributed to the study.