Eurasian blackcap migratory patterns are changing: before 1970 very few blackcaps spent the winter in the UK but they are now common winter visitors.
To understand this change, researchers from Oxford University and the British Trust for Ornithology (BTO) tracked 100 blackcaps (Sylvia atricapilla) across Europe. They discovered that the birds that overwinter in the UK are not behaving like other migratory species: instead of flying south for the winter, they are flying north. Our winter blackcaps have come from Spain, France and southern Germany.
The study found that the blackcaps that overwintered in the UK, returned to their European breeding areas up to 10 days earlier than those that wintered further south. This is behaviour that may be providing an evolutionary advantage, giving these early arrivals access to the best and most successful nest sites.
Other research from the University of Freiburg has found blackcaps that come north to winter in the UK show significant genetic differences from birds which spent the winter in southern Europe – even when both populations return to summer breeding areas in southern Germany. This finding suggests that the blackcaps that winter in southern Europe and those that spend the winter in the UK could be on their way to becoming two different species.
While climate change is the obvious driver of changing migration patterns, research suggests that the British habit of feeding birds is another factor. Birds are beginning to overwinter in Britain because our garden bird feeders are a regular and reliable source of food during hard times.
Header image: Blackcap male by Charles J. Sharp (CC BY-SA 4.0) flickr.com
Does this mean that the blackcaps that spend the winter here and the ones that come to nest here in the spring could become different species?
Speciation is a slow process. Apparently, established genetic differences such as the ones observed by the Frieburg study, can speed the process up but, on the whole, evolutionary timescales come in units of tens of thousands of years, if not millions.