During the winter, British bird-lovers lay out suet rolled with seeds and oats for birds landing in their chilly, barren yards. They believe that these handouts help the birds through winter, but scientists have now discovered that the practice may be splitting a species in two.
In the past 50 years, a migratory schism has appeared among central European blackcaps (Sylvia atricapilla). Native to southern Germany and Austria, the small, grayish songbirds used to fly southwest together to Spain to bask in the Mediterranean weather and dine on fruits such as olives. But in the 1960s, bird watchers started noticing some blackcaps wintering in the United Kingdom. These birds had split from their companions, headed northwest, and begun living off the feed left out by generous bird-lovers. Now, roughly 30 generations later, about 10% of blackcaps migrate to the United Kingdom in the winter instead of to Spain.
A recent study  showed that this new winter destination has led blackcaps to breed mainly with their migratory companions. The United Kingdom is closer to blackcap breeding grounds in central Europe than is Spain, so the northwest-migrating birds return home 10 days earlier and start mating among themselves.
Evolutionary biologist H. Martin Schaefer of the University of Freiburg in Germany and colleagues wondered whether the breeding separation had evolutionary consequences. So the scientists caught blackcaps when the birds returned to Germany in the spring and sequenced short stretches of their genomes called microsatellites. The genetic difference between the two groups is small but significant--on a scale from zero (totally similar) to one (separate species), they scored a 0.008. Despite this slight difference, the researchers could still use the genetic data to accurately assign 85% of the birds to the correct migratory group.
The scientists next compared the birds' physical traits. Northwest-migrating birds had rounder wings and narrower beaks, the researchers report  online today in Current Biology. These differences match the two groups' winter environments: The southwest-migrating birds' more pointed wings allow them to travel faster during their longer journey, and their wider beaks are better suited for swallowing olives and other Spanish fruits.
Although the two groups aren't geographically separated during the mating season, the migratory division has led to a rapid genetic and physical divergence between them as though they were reproducing kilometers apart, the researchers conclude. This split could be the start of speciation. But it's too early to know for sure, and the changes could always reverse if people in Britain stopped feeding the birds, Schaefer says. If nothing else, says Schaefer, the findings provide yet another example of the dramatic impact humans can have on other species. "Even [actions] based on good intentions, like feeding birds, can have evolutionary consequences on populations."
"[The study] has done a nice job of showing the early stages of speciation," says evolutionary biologist Michael Webster of the Cornell Lab of Ornithology. Whether blackcaps will eventually diverge into different species depends on the fitness of offspring from crosses between northwest- and southwest-migrating birds, says evolutionary biologist Darren Irwin of the University of British Columbia, Vancouver, in Canada. These hybrid birds are known to migrate due west, which would lead them to the Atlantic coast of France. If blackcaps wintering there fare worse than their counterparts in the United Kingdom or Spain, the "selection against [these birds] could promote further divergence between the two groups," Irwin says.