Becoming city birds
Songbirds provide evidence of evolution driven by urbanization
City dwellers everywhere often see rats scurrying behind dumpsters and have to sidestep pigeons feasting on abandoned sandwiches. Intrepid critters are a fact of urban life. But according to a new study published in Behavioral Ecology, some animals are actually evolving better adaptations to city living.
Researchers in California found key differences in urban and rural populations of a common songbird, the dark-eyed junco. The urban juncos allowed scientists to move closer before they took flight. They were also more willing to explore new habitats in captivity, and they produced lower levels of corticosterone, a stress-related hormone, than their rural counterparts after researchers handled them. In contrast, the rural juncos were more skittish around researchers and slower to investigate trees and food dishes in captive habitats.
Urban life provides a constant barrage of stimuli, demanding that birds be more flexible when foraging and nesting. The differences between these populations in behavior and physiology suggest that, when it comes to city living, it’s better to be bold.
“This urban habitat has created an opportunity,” said Jonathan Atwell, a post-doctoral researcher at Indiana University in Bloomington and the study’s lead author. “What we’re seeing is evolution in real time for them to expand their ecological niche to take advantage of these altered habitats.”
Until the early 1980s, the urban and rural juncos belonged to one population. The split occurred when a small flock of migrants wintered at the University of California, San Diego, according to earlier research. When spring came, the juncos remained on campus instead of returning to the nearby mountains.
In the new urban environment, natural selection favored assertive birds. These braver juncos were more likely to pass on their genes, including those correlated with bold behavior and lower corticosterone levels. Over time, the authors propose, the population evolved and boldness became the norm. Similar changes are being observed elsewhere in America and Europe as other groups of urban birds become bolder.
But some researchers are less sure about that interpretation. Niels Dingemanse, a songbird researcher at the Max Planck Institute for Ornithology in Seewiesen, Germany, cautions against drawing conclusions based solely on the contrast in habitat. “The study may be phrased in the context of urbanization,” he noted, “but there is no statistical evidence showing that the population differences are not due to any other factor,” such as different population densities or predation pressures.
But Atwell believes that more evidence for natural selection will come with time. “We collect blood samples from these birds and have their genomes,” he said. “When the technology catches up… we can go back and ask these questions in more detail.”
In the meantime, Atwell and his colleagues know where to find their dauntless subjects—the juncos have started nesting in bike helmets and flowerpots.
Atwell’s research and the juncos’ move to the city are featured in a documentary film premiering December 2012. Those interested can check it out at www.juncoproject.org.