Until recently, we knew the blackcap as a summer visitor - leaving for southern Europe or north Africa in September.
Since the 1960s, the number of blackcaps which spend winter in the UK has grown and grown. It's no longer a rare sight to see them in your garden in the middle of winter.
Just what are they doing? Surely blackcaps should be heading for warmer climes? After all, the UK's no place for a warbler in winter...
We've known for a while that the blackcaps that come to Blighty for winter tend to have been hatched or breed in southern Germany. We found that out from ringing, where birds are fitted with a uniquely-numbered, lightweight metal ring which can be read and reported if they're found or caught again.
Martin Schaefer and his colleagues at the University of Freiburg have been studying two blackcaps in two areas of Germany, 500 miles (800 km) apart. Birds which spent winter in Spain had more in common genetically with their Spanish sun-loving counterparts in the other population than they did with their UK-wintering neighbours which bred in the same area!
The scientists reckon that it could mean that the Spanish and UK-wintering groups of blackcaps could be on their way to becoming two different species. And the reason? People in the UK putting out plenty of bird food have made spending the winter here a viable option for blackcaps.
How you can help
If you see a blackcap during winter, please add details of your sighting to Birdtrack, a BTO, RSPB and Birdwatch Ireland project. By collecting information about where birds are when, we can learn more about their changing behaviour and populations.
By putting out fruit (try putting an apple in the branches of a bush or tree), fat and even seeds, you could tempt a blackcap into your garden.
Blackcaps can be quite feisty at bird feeders and tables, so watch out for them chasing away other birds! By providing several sources of food in different bits of your garden, you should be able to avoid too many feathers flying.