In the classic Aesop tale of the city mouse and the country mouse, the country mouse’s life is highlighted by simple food and quiet company, while his city cousin’s fancy dinner may have had better food but was interrupted by nasty dogs. City birds and country birds also live different lives, though there’s no fable to illustrate the dissimilarities. Two new studies identify some of these differences and make startling conclusions about what they might mean for avian species.
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In the first study, published in Behavioral Ecology, researchers from the Smithsonian Conservation Biology Institute and elsewhere studied the effect of urban development on birdsong, focusing on seven species of songbirds—including northern cardinals, American robins and house wrens—in 28 sites in rural, urban and in-between areas across the Washington, D.C. and Baltimore, Maryland region. (The study sites were all part of the Smithsonian Neighborhood Nestwatch citizen science project.) At each site, the researchers recorded bird songs and characterized the levels of urban development and ambient noise.
“In order to survive and reproduce, it is imperative for birds to be able to transmit their signals to one another,” says study co-author Peter Marra, a Smithsonian ecologist. Birds use their songs for a wide variety of functions, such as recognizing other members of their species, attracting mates and defending territory. “Now it seems they may be having trouble in urban areas,” Marra says.
The researchers found that in noisy areas, birds tended to sing notes in higher pitches; city noise can mask lower pitched sounds. In areas with many buildings and hard surfaces that reflect and distort higher pitched sounds, the birds often compensated by singing songs with deeper tones. But birds that had to deal with both noise and buildings had difficulty compensating for both factors. And if they cannot figure out a way to do so, these birds may have bigger problems. “We now need studies to determine if these changes in song translate into differences in reproductive success,” Marra says.
The second study, published in Oikos, compared urban and rural populations of the European blackbird (Turdus merula) in a 1,700-mile transect from Spain to Estonia. This blackbird was once found only in forests but has adapted well to the urban lifestyle and is now one of the most abundant birds in European cities within its range. These birds should migrate south in the winter, but the researchers say that not all of the birds are doing so.
The scientists looked at markers of migratory behavior in the birds’ feathers and claws and found that the urban blackbirds were more likely to be sedentary and not migrate. Why? Urban areas are warmer than rural ones (because of the urban heat island effect), so the city birds are able to escape the worst effects of winter without taking the trouble to fly so far. In addition, they may also be able to take advantage of supplementary food (such as bird feeders) that would help them get through the colder months.
By not migrating, the urban birds may be able to breed earlier and also avoid the higher numbers of parasites migratory birds encounter, the scientists write. In addition, the sedentary birds may breed with the migrating population less often. All those factors may eventually add up to the urban bird populations evolving into separate species from their country cousins.