More than a century ago, some European Blackbirds gave up the commuting life. The traditional routine was to nest in northern forests but head for southern Europe or northern Africa at the first sign of winter. Then some populations discovered that winter in the city isn't half-bad: The microclimate is warm with plenty of tasty leftovers. So strong is the appeal of city life, according to a research team in Germany, that it is has not only changed the blackbirds' behavior, but their genetics, too.
"It's a very cool study, with a simple message: Urbanization is an important evolutionary force," says Roarke Donnelly, an ornithologist at Oglethorpe University in Atlanta, Georgia. "We've been thinking about this stuff for a long time, but it isn't easy to test. And they've done it with a simple but elegant experiment."
Jesko Partecke, an ornithologist at the Max Planck Institute for Ornithology in Erling, Germany, knew that European blackbirds (Turdus merula) in Munich stayed put in winter while ones living nearby forests still migrated. To see if the urbanized birds had evolved, Partecke and Eberhard Gwinner (now deceased) collected chicks from the two settings in the spring of 1998. They raised them in their lab in individual cages with light and temperatures mimicking Munich's. As summer gave way to fall, and winter to spring, they recorded the birds' nocturnal activity; this "migratory restlessness" is inherited and correlates with the distance a bird travels on its migrations.
The urban males were the least active of all the birds, preferring to sit quietly in their cages while other birds hopped about. In contrast, the urban females were just as active at night as their forest counterparts, indicating that the Munich females continue to migrate. "We were completely surprised by the females," says Partecke. "We naturally assumed that both males and females had changed." The difference may be bullying; males, which are larger, are known to drive the females away from food and warmth, says Partecke. As a result, he speculates, city-females who try to stay in town through winter may end up dying.
Another benefit of city life for males is that they reach sexual maturity earlier there than in the forest. Because blackbirds have multiple broods, this means they may have more offspring. "The shift to being sedentary seems to be adaptive in urban habitats," the authors say.
The findings are described in this month's Ecology. "It's good to see a study that's looking at the evolutionary pressures caused by these pseudotropical bubbles, our cities," says Eyal Shochat, an urban ecologist at Arizona State University in Tempe.