By Victoria Gill
Science reporter, BBC News
Birds that spend winter in the UK find food in many gardens
Bird-feeders, hung in many a garden, can affect the way our feathered friends evolve, say scientists.
European birds called blackcaps follow a different "evolutionary path" if they spend the winter eating food put out for them in UK gardens.
The birds' natural wintering ground is southern Spain, where they feed on the fruits that grow there.
Researchers describe the impact this well-intentioned activity has had on the birds in Current Biology journal.
Dr Martin Schaefer from the University of Freiburg in Germany led the research.
He and his team found that blackcaps that migrated to the UK for the winter were in the very earliest stages of forming a new species.
He explained that some blackcaps (Sylvia atricapilla) would always have migrated "a little further north" than others and eventually "ended up in Britain in the winter".
"But those birds would have had nothing to eat," he said.
It was when garden bird feeders became more popular in the UK, that an evolutionary division began to emerge.
"As soon as the British provided a lot of bird food, those birds would have had a much higher probability of surviving the winter."
And because the UK is closer to their breeding ground, those birds would also have returned earlier to claim the best territory.
The researchers, from Germany and Canada, set out to discover if the birds that spent the winter availing themselves of garden bird-feeders were in fact a distinct group.
To do this, they studied the blackcaps at a breeding ground in Germany.
The team were able to use a chemical "signature" from the birds' claws to identify where they spent the winter, and what food they ate.
"Then we took blood samples and analysed those to assess whether... we had two distinct populations. And that's exactly what we found," said Dr Schaefer.
The blackcaps have adapted to their shorter journey and different food supply
"To a very large extent the birds only mate [with] birds with the same overwintering grounds as them."
This initial "reproductive isolation", Dr Schaefer explained, is the very first step in the evolution of a new species.
"This tells us that by feeding birds in winter we... produce an evolutionary split. And we have produced these initial steps in as little as 50 years."
The team also observed differences in the birds' beaks, wings and plumage.
Blackcaps that migrated along the shorter route to the UK had rounder wings, and longer, narrower beaks.
The scientists said these differences were evidence that the birds had adapted to their shorter journey, and to eating seeds and fat from bird-feeders, rather than fruit from shrubs and trees.
But, Dr Schaefer pointed out that the evolution of a new bird species "could take 100,000 to a million years".
"At this stage this is reversible," he added. "And it's hard to envision a species change, because if there's another economic crisis and people stop feeding the birds, the whole system might just collapse."
In this case, Dr Schaefer thinks the human impact on blackcaps has been a positive thing.
"[The birds have] found a better overwintering area that is closer to the breeding ground, where they can obtain food easily.
"And I also think its positive news for us, because it means not all the changes we produce are necessarily bad, and that some species have the potential to adapt quickly to the changes."
Grahame Madge from the Royal Society for the Protection of Birds (RSPB) said that this was "a fascinating piece of research" and that it fitted in with the birds adapting to a changing climate.
"Blackcaps have been able to start this behaviour because of the milder winter we've experienced in the last few decades," he said.
"And because they're getting food, this reinforces the behaviour and will enable them to survive a colder winter [in the UK]."
Joseph Tobias, a biologist from the Oxford University in the UK, agreed that the UK climate may have been a more important factor contributing to the changes observed in the blackcaps.
"The study clearly demonstrates that a new lineage has arisen... [but] it doesn't actually demonstrate that food hand-outs by humans are the root cause," Dr Tobias said.
"It is possible that the main reason for the switch in migratory behaviour was a warming winter climate in the UK. The best we can say on the basis of the evidence is that the increase in bird-feeding in the UK may have contributed to the switch in behaviour."
Mr Madge added that putting food out for birds in the winter was "very important" and that many birds "need the energy boost at this time of year".