Breakaway Blackcaps

Blackcaps are not an uncommon sight in Britain during the winter months nowadays. We have a ‘healthy’ wintering population of around 3,000 individuals which compares with a summer population estimate of 932,000 pairs (Birdlife International, 2004). Numbers wintering in the UK have increased considerably since the 1960s and it would seem likely that the above number is an old and conservative estimate so if anyone has an up-to-date figure I would be pleased to receive it.

Numbers wintering in Northants appear to have risen and averaged higher since the early 1990s, although this may reflect better observer coverage and communication.

From the early 1990s numbers are approximate, based largely on localities and estimates rather than specific individuals. Source: Northamptonshire Bird Report 1969-2010.

Ringing data have established that UK winterers are from a breeding population in central Europe and, while most Blackcaps from this area head south-west in autumn to winter in Spain and northern Africa, some head north-west to the UK. The driver for this is not clear but milder winters and an abundance of ‘artificial’ food (i.e. winter bird feed provided by man) have been cited as the likely reasons. The latter of the two factors appears to be the subject of debate as many observations on wintering Blackcaps point to their feeding primarily on natural food sources such as berries and insect larvae when these are available.

Male Blackcap, Northampton, December 2012 (Dave Warner)

Dave Warner’s excellent portrait of a male in his Northampton garden last weekend illustrates this point perfectly with the principal choice of food, Dave suggests, being an abundant supply of crab apples. That said, it seems logical to assume that ‘artificial’ food may at least act as a regular supplement or backup in the absence of natural foods, thus helping maintain the wintering population.

What is clear, however, is that our wintering Blackcaps are very different from those which breed here in summer. Having a shorter distance to fly to their breeding area in spring means they arrive back before the Spanish winterers and therefore only have each other to mate with, effectively becoming reproductively isolated from the Spanish birds.  So now there are morphological differences emerging. This population, in the space of little more than 50 years, has produced birds with rounder wings (as they don’t need to migrate so far) narrower and longer bills (supposedly for taking advantage of ‘artificial’ foods) and browner mantles and bills. A kind of ‘catalytic evolution.’ How far will it go? Full speciation?

So next time you come across a Blackcap in Britain in winter it’s worth remembering that it’s not just any Blackcap, it’s likely to be a breakaway Blackcap – an activist, a rebel, a pioneer and, although a potential champion of speciation, it may only ever become genetically distinct to a racial level, so don’t hold your breath for an ‘armchair tick’ any time soon …

For a more detailed look at this unfolding phenomenon see here and here.