In the forests of Germany live large numbers of blackcaps, a small species of songbird. They all look very similar, but they actually belong to two genetically distinct groups that are becoming more disparate with time. For the moment, the best way to tell them apart is to wait for winter. As the cold sets in, one group of blackcaps flies southwest to Spain, while a smaller group heads northwest towards Britain.
If the prospect of spending winter in Britain rather than Spain seems odd to you, you're not alone. Indeed, blackcaps were hardly ever ventured across these shores before the 1950s. But since then, the birds have taken advantage of the glut of food left out on bird tables by animal-loving Brits. These banquets, along with the luxury of not flying over the Alps, have made Britain an increasingly popular holiday destination for wintering blackcaps. And that has set them down the path towards becoming two separate species.
The mystery of Britain's winter blackcaps was solved in a classic series of experiments by Peter Berthold (awesome beard) in 1992. Berthold found that chicks from the two populations (those that fly to Britain and those that fly to Spain) would always fly in the same direction as their parents even if they were raised in identical environments. This strongly suggested that their travel plans were genetically set, and Berthold proved that by breeding birds from the two groups. Amazingly, their offspring migrated in a west-northwest direction, about halfway between the routes of their parents.
Berthold went on to show that the blackcaps' inherited itineraries were the result of a handful of genes at most. And these initial differences have become magnified over time. When spring returns, the blackcaps fly home, they select mates and they form bonds that will last until the next year. But those returning from Britain have less distance to cover so they reach Germany first and they pair up with each other. When the stragglers from Spain get there, they only have each other to mate with.
Even though all of these birds spend most of the year in each others' company, they are actually two populations separated by barriers of time that prevent genes from flowing from one group to another. Gregor Rolshausen from the University of Freiburg has found that their genetic separation is already well underway.
He has found the Spanish migrants are genetically more distinct from the British ones than they are to individuals from more distant parts of Germany, some 800km away. These differences have arisen over just 30 generations and they're now sizeable enough that with a bit of DNA sequencing, individuals can be assigned to the right group with an accuracy of 85%.
It's highly unlikely that the British migrants arose because of an influx of genes from other blackcap populations. For a start, no European blackcaps had ever been found to migrate in a northwesterly direction before 1960.
Instead, Rolshausen thinks that the crucial factor was human altruism - by giving food to wintering birds, we also gave an advantage to any individuals with mutations that sent them in an unorthodox direction. Previously such birds would have simply died, but with humans around, they (and the genes they carried) flourished.
Their bodies have even changed. The British migrants have rounder wings. In general, European blackcaps with shorter migration routes tend to have rounder wings - they're more manoeuvrable but less suited to long distances. They also have narrower and longer beaks, for they are generalists that mostly eat seeds and fat from garden feeders. Birds that arrive in Spain eat fruit and those with broader bills can eat larger fruit.
Their colours are also slightly different. British migrants have browner backs and beaks, while the Spanish migrants are greyer. It's not clear why, but Rolshausen thinks that these changing hues could provide the birds with a way of recognising, and sticking to, their closer relatives.
This is one of the few studies to show that human activities - the provision of food to wintering birds - are powerful enough to set up reproductive barriers among animals that live in the same place. It also shows that these first few steps of speciation can happen with extraordinary pace, in just 50 years or so. As Rolshausen notes, the blackcaps are testament to the speed with which evolution can operate.
No one can say whether the blackcaps will actually split into two different species. All the conditions are right, but our activities may change the playing field once again, so that the birds experience entirely new sets of evolutionary pressures.
Reference: Current Biology 10.1016/j.cub.2009.10.061
More on speciation:
- Discriminating butterflies show how one species could split into two
- New plant species arise from conflicts between immune system genes
- Giant insect splits cavefish into distinct populations
- How diversity creates itself - cascades of new species among flies and parasitic wasps
Yet to see a blackcap on any of my bird feeders, but we've still got several months of winter to go.
Quite funny. IÂ´m German and read about German research in English here on your blog. Great article as usual!
There is another bird, it is called Rotkehlchen with an organge (or red) breast. These birds also left Germany during the winter, but now they stay because of people, who feed them.
I often wondered what influences feeding might have on critters.
"Rotkehlchen" = robin, for anyone who's wondering...
Why do they have to cross the Alps to fly to Spain? I would have thought they have to cross the Pyrenees (and they should cross the Alps to go to Italy). Cool post though!
Do they carry coconuts with them as they migrate (for all you Monty Python fans!)?
(Ed- there's a "were" in the second paragraph that doesn't belong).
Nice piece. Looks like it could become another case of sympatric speciation.
Neat find. I wonder if this finding would be a good opportunity to study genetic assimilation... it might be a reason they diverged so quickly.
Even odder actually, because Britain has its own summer population of Blackcaps which migrate to Spain in the winter to be replaced by German birds. For several years it was thought that the winter Blackcaps in the UK were birds that bred here in the summer and remained throughout the winter. Wrong. The Brtish population goes to Spain in the winter (Blackcaps haven't been affected by the strength of the Euro in the same way as Homo sapiens)and part of the German population moves here.
I was just reading about Black Caps in Cyprus being caught in poacher's nets and sticky lime sticks for sale to tourist cafes.
(Never heard of 'em before.)
Great write up Ed. I think this is one of the coolest evolution stories of the year.
Many thanks for the link to my page on Ambelopoulia.
Great article, but a question as someone who hasn't gotten a chance to download the article yet and read it - what are the Brits feeding the incipient species with? Are they feeding the insect or seed/fruit part of the Blackcap diet?
Not sure if I'm supposed to feel good or bad after reading this! I'll be the first to admit I'm a Birdfeeders.com addict! But personally I think the changes in migratory patterns are natural and would happen even if there weren't humans, food sources come and go on their own as well.
'They also have narrower and longer beaks, for they are generalists that mostly eat seeds and fat from garden feeders'.
I do have some trouble with this research. Statements such as that above are far to general. Birds within any species can show size differences when subjected to measuring like this. By that I mean one would need to take measurements from a large sample of birds in other countries, as well as large samples from here, as we now know that most of the birds that winter here are from the continent. There may well be a geographical size difference notwithstanding birdfeeders. I simply can't see the connection to bird feeders either. I fail to understand how that connection has been made. That having been said it is an interesting article in any event.
We have a pair of blackcaps in our garded, the male first sited 15th Dec 09. My husband and I have been putting on the bird table the usual bird seeds but when we add apple we have noticed the male blackcap becomes very aggressive with sparrows,robins,chaffinch and dunnocks, not allowing them to feed. Is this their normal behaviour?