Posted: Thursday 5th April 2018 by Twheatear

Blackcap pair

Watching evolution in action...

The recent snows have brought some new birds to our garden feeders – blackcaps. The bird book I had when I was young says those warblers are summer visitors to the UK, with a particularly beautiful song. We still have to wait a few weeks to hear the song, but since the 1960s, an increasing number of the birds have been here in winter too.

But ringing studies have revealed that these aren’t the same individuals. The ones that will be with us in summer still spend their winters in Spain. The birds that are here now will soon be heading off to their nesting areas, probably in central Europe. Normally those continental birds head south-west for the winter but a few, whose sense of direction was a bit “off”, come west to the UK instead. In the past, that was likely to be a fatal mistake, with few surviving our winters. But their chances of survival were improved when humans started providing food, and enough seem to make it back to breed that there’s now a significant number with the westerly habit.

Even more remarkable, migration direction turns out to be an inherited characteristic. So the offspring of UK-wintering birds are also more likely to come here. That tendency is enhanced by the fact that the continent is a shorter flight from the UK than it is from Spain, so UK-wintering birds are more likely to form pairs when they arrive together, early, on the breeding grounds. Spain-wintering birds arrive a bit later, so are also likely to pair up together. Even though both groups of birds nest in the same place, it’s now possible to detect separate genetic groups: an early sign of one species splitting into two. One day, UK birders might be able to record two ticks a year on their species lists: EastWest Blackcaps in winter and NorthSouth Blackcaps in summer.

[Thanks to our friend for the great picture ]

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