Evolution in Progress - Eurasian Blackcap

Male Blackcap, Sylvia atricapilla

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It's nice to be able to report on a recently-observed example of behavioural change in one of Britain's more tuneful warblers - the Eurasian blackcap - brought about by human activity.

When I was young, a popular ornithology trivia question was, what is the only resident British warbler? 'Resident' meaning non-migratory and remaining in Britain all year round.
The answer, found after hours of searching though my bird books, was the Dartford warbler, a very pretty little warbler found in southern England and believed to then be the only warbler to over-winter here.

That's now changed. As I witnessed last winter with one regularly coming to feed in our garden, and again a few days ago when I saw and heard one singing in a tree in my sister-in-law's garden, an increasing number of Blackcaps are now resident in Britain throughout winter, at least in Oxfordshire. There have also been numerous reports from other parts of the UK. This has led scientists to speculate that this could be the beginning of a speciation as the two populations become reproductively isolated.

Female Blackcap, Sylvia atricapilla

Normally, most of our warblers - whitethroats, chiffchaffs, garden warbler, wood warbler, nightingales, etc, migrate south for the winter to Iberia or North Africa, returning in spring to breed. Indeed, the call of the chiffchaff and the singing of the warblers is as much a harbinger of summer as is the call of the cuckoo and the sight of swallows.

However, this is probably not quite what it seems in that this is not native-born Blackcaps over-wintering here. Most, if not all of our native-born Blackcaps still seem to head south to Iberia in autumn but they are replaced by an increasing number of migrants from Southern Germany which, instead of also heading southwest to Iberia, migrate west to Britain.

The reason for this change appears to be the availability of food due to the British practice of having bird feeders in gardens to attract songbirds with seeds and fat-balls. This practice of feeding birds, especially in winter, has already been suggested as the cause of a measurable increase in the bill length of British Great tits compared to the bill of those in the Netherlands, as they adapt to feeders designed to be inaccessible to larger birds. Britons spend about twice as much on wild bird food as other Europeans to the extent that feeding birds is almost a national pastime.

This is a nice example of the speed of evolution. It is something that we can see with our own eyes if we only look closely enough. It doesn't have to take millions of years.

Martin Schaefer

Researchers, Martin Schaefer and his colleagues at the University of Freiburg believe there is a genetic difference between the population which over-winters in Iberia and those over-wintering in Britain. In earlier times, any predisposition in Blackcaps to migrate west instead of south would have resulted in their removal from the gene-pool. Now, however, it results in a healthy individual fit and ready to return 'home' to breed, fed on peanuts and other seeds by bird-loving Britons.

The warblers are the smallest of a large group of songbirds that includes the chats (of which the European Robin is one), wheatears and thrushes. Blackcaps, Sylvia atricapilla, the females of which have chestnut brown, not black, heads are closely related to garden warblers, Sylvia borin although they have a different appearance but very similar songs. Blackcaps are highly territorial and will try to drive out not only other Blackcaps from their territory but also any garden warblers present.

This behaviour is an evolved prezygotic barrier to hybridization in two closely related species that probably could interbreed otherwise. Rather than their songs diverging significantly, they have evolved different plumage and an aggressive territorial response to one another.

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