I currently have a male Blackcap (Sylvia atricapilla) in my back garden, he has been there for at least two weeks now, probably longer. Every time I see him he is either rummaging in the dense ivy that entwines a Birch tree or is fluttering about in a flowering Camellia bush. I have even heard him utter some short phrases of sub-song on a sunny day. Blackcaps are migratory warblers; birds that breed in Britain during the spring and summer, then head south to the Mediterranean or North Africa to spend the winter (nice for some). So why is there one in my garden in February?

The phenomenon of Blackcaps overwintering in the UK is no longer shocking news, it isn’t even a ‘phenomenon’ anymore – it is becoming the new normal. It was first noticed in the 1960’s, although it had likely been happening for hundreds of years before then on a smaller scale. It would seem that thanks to a warming climate, which has resulted in milder winters in recent decades, insectivores like Blackcaps and Chiffchaffs that were unheard of in winter before, can now survive here fairly easily.

But thanks to ringing recoveries we now know that the Blackcaps wintering here are not the same birds that breed here in the summer, they are largely individuals from continental Europe. Extensive research in the last few years (which is ongoing), has revealed that the majority of wintering birds are from central Europe – countries such as Germany, Belgium and the Netherlands, although there are also birds from Scandinavia (more on those later).

Theories behind why this new migration route has developed include the idea of reverse migration – by which birds fly in the opposite direction to where they should be headed due to genetic errors/mutations. As some Blackcaps migrate in a south-easterly direction towards the eastern Mediterranean, it could be that some of these are flying north-west to Britain. Another idea is that the new route is a widening northwards of the usual south-west migration of most European Blackcaps. As for birds seemingly originating from Scandinavia, research (Kopiec & Ozarowska 2012) has strongly suggested that these individuals are in fact central-European birds that have reverse-migrated northwards and then are re-orientating south-west and ending up in Britain.

The main reason that Blackcaps are able to survive in the UK during winter at all is because of our comparatively mild climate, which has gotten milder in recent decades, look at a map of wintering Blackcap records and there is a heavy bias to the south-west of the country. Another reason that has been suggested is that our provisioning of bird food has helped survival, (this is not a cause of the new migration route as some media has reported, it merely aids survival) something I am not sure of.

gbw-blackcap-reporting-rate-in-january-2013
BTO map of wintering Blackcap distribution in 2013.

The Blackcap in my garden has never approached or been seen on the feeders hanging from a tree in my front garden, he seems to feed exclusively in local evergreen shrubs or ivy – presumably for invertebrates. Other wintering Blackcaps I have seen in previous years were in the countryside, far from any feeders. No doubt some Blackcaps do take advantage of human-provided food sources, but I am wary of the claim that it is a major factor in their survival here. Perhaps most birds are seen in gardens not because of feeders, but because of the presence of many evergreen shrubs and trees – which likely harbour a lot of hibernating invertebrates.

Where this migration shift could lead – who knows? If Britain continues to have (increasingly) mild winters to ensure their survival then this splinter-population could continue to diverge further genetically from their cousins who love the Med. Or they might not, after all they still share breeding grounds. Either way this is a fascinating development, which no doubt has further intricacies to be revealed by more research. Plus it’s nice to have a reminder of a warmer time of year living in your garden through the ‘grey months’.

If you are interested here is a link to a scientific paper from the journal ‘Ornis Fennica’ which goes into detail about a study into the source of overwintering Blackcaps in Britain; http://www.ornisfennica.org/pdf/latest/3Kopiec.pdf

Advertisements