Birds of a feather flock together — right? Not necessarily, according to new findings from researchers in Germany. Thanks to bird feeders, some birds may even be evolving into different species. A study out of the University of Freiburg shows Central European blackcap warblers have split into two separate reproductive groups because an abundance of bird feeders in the U.K. allow about 30 percent of the birds to spend winters there. 

The changes in the birds have been drastic, showing the evolution can move quickly. In fewer than 30 generations, birds visiting Britain have evolved different-shaped wings and beaks. And now, what was once a single population of birds known as blackcaps is now two reproductively isolated groups.

Blackcaps have historically migrated from Germany and Austria to Spain during the cold months, and any birds that tried wintering too far north died from starvation. But this new winter territory has shaved 360 miles off their traditional 1,000-mile migration to Spain.

Dr. Martin Schaefer of the University of Freiburg in Germany led the study. As he told reporters, “The new northwest migratory route is shorter, and those birds feed on food provided by humans instead of fruits as the birds that migrate southwest do.” And this new, dependable wintery diet allows for evolutionary tweaks in the bird’s bodies.

Consequently, physical differences between the two sets of birds are emerging. The blackcaps migrating northwest have rounder wings. This provides better maneuverability but makes the birds less suited for long-distance migration. They also have longer, narrower bills that are not so good for eating food such as Spanish olives during the winter. As Schaefer says, “It shows the profound impact of human activities on the evolutionary trajectories of species.”

Schaefer points out that, at this stage, the process is still reversible. If people were to stop feeding the birds, (perhaps due to economic hardship) the whole system might collapse. The complete evolutionary process could take 100,000 to a million years. 

In other words, bird watchers shouldn’t update their guidebooks just yet.