Bird-feeding is considered a great pleasure for the humans who do it -- promoting an interest in and concern for our feathered friends. It's also considered great for the birds, who may need a bit of help finding food in the region of our overly pristine suburban lawns.

But a group of researchers based in Canada and Germany have found that feeding birds can effect their evolutionary trajectory. Publishing their study in yesterday's edition of the journal, Cell Biology, they showed that feeding a group of "blackcap" birds resulted in their split into two "reproductively isolated" populations within 30 generations. 

The birds continue to breed in the same forests, all but side by side, but just not with birds from the different group.

"Our study documents the profound impact of human activities on the evolutionary trajectories of species," Martin Schaefer of the University of Freiburg said in a prepared statement. "It shows that we are influencing the fate not only of rare and endangered species, but also of the common ones that surround our daily lives."

According to a press release about their work: The split that the researchers observed followed the recent establishment of a migratory divide between southwest- and northwest-migrating blackcap (Sylvia atricapilla) populations in Central Europe after humans began offering food to them in the winter. The two groups began to follow distinct migratory routes—wintering in Spain and the United Kingdom—and faced distinct selective pressures. Under that pressure, the two groups have since become locally adapted ecotypes. (Ecotypes represent the initial step of differentiation among populations of the same species, the researchers explained. If ecotypes continue down that path, they can ultimately become separate species.)

"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," said Schaefer. "As a consequence, birds migrating northwest have rounder wings, which provide better maneuverability but make them less suited for long-distance migration." They also have longer, narrower bills that are less equipped for eating large fruits like olives during the winter. 

According to the press release, Schaefer says it isn't clear whether the ecotypes will ever become separate species; in fact, he doubts they will because the habits of humans will tend to change over time. Even so, the findings do speak to the long-standing debate about whether geographic separation is necessary for speciation to occur. In particular, it had been contentious whether selection could act strongly and consistently enough in sympatry to separate a united gene pool.

"In highly mobile organisms such as birds, the consensus is that sympatric speciation is extremely rare, mainly because it is difficult to envisage how gene pools could be kept separate until speciation has occurred," Schaefer said. "Our results now show that the initial steps of speciation can occur very quickly in a highly mobile, migratory bird," because divergent selection during the overwintering phase leads to the evolution of reproductive isolation."