Until now, most people have likely regarded bird-feeders as merely a pleasant addition to their gardens. But scientists have now discovered that bird-feeders in the UK are actually having a serious long term impact on bird life – they've found that the feeders have brought about the first evolutionary step in the creation of a brand new species.
Historically, European Blackcap birds migrate to Spain to spend their winters, where they feed on fruit and berries. While in the past the part of the population that accidentally flew to the UK had a hard time surviving, since the rise of bird–feeders in the UK things changed.
The food supplied by animal-loving Brits, 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, Gregor Rolshausen from the University of Freiborg and colleagues write in the journal Current Biology.
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. 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. The differences between the two groups are large enough that with a bit of DNA sequencing, individuals can be assigned to the right group with an accuracy of 85% – they have arisen over merely 30 generations.
Rolshausen and colleagues think that the crucial cause of the split was caused by humans giving food to wintering birds, which 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) survived.
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 are more maneuverable and 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 colors are also slightly different. British migrants have browner backs and beaks, while the Spanish migrants are more gray. Researchers suggest that these changing hues could provide the birds with a way of recognizing, 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. The development of the blackcaps show the speed with which evolution can operate.
Time will tell whether the blackcaps will actually split into two different species. All the conditions are right, but human activities may change the playing field once again, so that the birds experience entirely new sets of evolutionary pressures.