Not Flying South

Backyard feeders plus a strange sense of direction may have begun to split one bird species into two.
In southern Germany, some 10 percent of blackcaps (Sylvia atricapilla) fly not south, toward warmth, but rather northwest for the winter, says evolutionary biologist H. Martin Schaefer of the University of Freiburg in Germany. This novel journey, on record since the 1960s, probably became survivable thanks to the rise of backyard bird feeding in Britain, he says. Enthusiasts setting out seed and suet have kept the birds from starving until it’s time to wing back to Germany to nest.
The returnees from Britain nest in the same German forests as the more conventional migrators that fly to Spain. Yet the two groups now show subtle but distinct genetic and visible physical differences, Schaefer and his colleagues report online December 3 in Current Biology.
“It’s a good example of how humans can influence evolutionary trajectories,” Schaefer says.“The really cool thing here is that it seems to be driven by migration,” says behavioral ecologist Jeff Podos of the University of Massachusetts in Amherst. Other researchers have looked for these kinds of genetic differences between populations with different migration destinations, but have not found any, he says.
One or just a few genes are thought to control the migration direction for blackcaps, Schaefer notes. A classic study showed that offspring of parents that flew in divergent directions for winter grew up to migrate in an intermediate direction.
Migratory genes would, of course, differ between differently migrating populations, but Schaefer and his colleagues wanted to look for other genetic differences as well. Northwesters and southwesters varied slightly in parts of their genome that don’t involve migration. Small as that difference was, it was greater than the difference between southwest migrators hailing from far-flung parts of Germany, says Schaefer.
Northwester birds also had slightly rounder wings than the southwesters, the researchers said. Rounder wings improve flight maneuverability but don’t perform as well on long hauls. The northwester birds fly only two-thirds the distance the others do, so the slight shift in wing shape might be the beginning of adaptation to their new route.
Likewise, beaks on Anglophile blackcaps tended to take on a different shape, one more like the narrow beaks on generalist feeders. Schaefer speculates that winter feasting on olives and other big, fruity mouthfuls encourages wider beaks in the Spanish migrators. These blackcaps are more likely to choose mates that have similar winter tastes, thus preserving migratory differences in their offspring. The two bird groups aren’t new species yet, Schaefer says. But they’ve taken a few flights in that direction. More than 50 species that have recently rerouted their migrations may be experiencing similar changes, Schaefer says.