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Ground-nesting birds like the ovenbird, above, and some thrushes can be extremely sensitive to urban pressures.
Are cities for the birds? Yes and no.
Why some birds just can't hack it in the big city
By Deane Morrison
December 1, 2005
Just like people, some birds are city slickers and some are country bumpkins. As urbanization spreads, we see more "city" birds like crows and pigeons and fewer "country" birds like warblers, for whom the city life holds little appeal. Robert Blair, an associate professor of fisheries, wildlife and conservation biology who has studied urban birds for several years, calls city birds urban exploiters, for their ability to exploit this relatively new type of environment. Their opposite, the urban avoiders, can't thrive in a city environment. Here's why. For one thing, cities offer a variety of new foods. Birds limited to a diet of insects often can't find enough in the asphalt jungle. Instead, the less picky omnivores, such as starlings and sparrows, come in and feast. Another factor is the stressful brooding environment of a city. "It's apparently harder to fledge a brood in an urban area," says Blair. This is a big problem for warblers that migrate to the tropical areas of Central and South America for the winter. The migration takes a lot of time and energy, leaving little of either for raising chicks during the breeding season. If a pair loses their first clutch of eggs, that's it; they're done for until the next year. Therefore, successful members of those species will be ones that breed in more egg- and chick-friendly environs outside the city limits.
Crows obviously do well in cities, and one reason may be their neophilia, or love of new things."But the exploiters [are multiple brooding species]. I think that's why some birds are successful-- those with multiple broods do better in an urban environment." If a storm wipes out a nest, a multiple brooding species can have another clutch of eggs and get another chance at leaving offspring.
Why fledging is harder in cities isn't clear, says Blair. Some scientists think there's more predation, but he hasn't found any evidence of this. In fact, after following the fates of cardinal and robin nests in an area similar to Dinkytown (a mixed commercial/residential area adjacent to the Minneapolis campus), he found less predation than in rural areas. Fewer predators? Perhaps, although places like Dinkytown have their share of raccoons, bluejays, and crows--egg-eaters all. Ground-nesting birds like ovenbirds and some thrushes can be extremely sensitive to urban pressures. Even something simple like a walking trail through a natural area can disturb such birds. Not only do trails bring the disturbance of human visitors, but raccoons and other predators use trails to get around more efficiently. Cities, with few untrodden areas and lots of pets, are not for ground-nesters. Crows obviously do well in cities, and one reason may be their neophilia, or love of new things. One of Blair's graduate students is now investigating the idea that this trait, akin to curiosity, is the main reason why crows have so successfully invaded the city. Some city birds may be just plain pushy. There is some evidence from other researchers, says Blair that starlings looking for good nest cavities in trees may displace woodpeckers from their holes. But between the exploiters and avoiders of cities lies at least one more type of bird: those that find the city a nice place to visit, even though they wouldn't want to live there. Another of Blair's graduate students is finding that some birds--for example, blackpoll warblers--seem to take "pit stops" in urban areas situated on rivers. The waterfront doesn't have to resemble a pristine river at all; any old river shoreline will do for a blackpoll on migration. The pattern of certain birds "making it" in the city and others disappearing with urban sprawl is a common one. Blair, who has been at the University for three years, studied birds in southwest Ohio for many years and found the same thing. "It's a pattern that's predictable for any large metro area," he says. Even so, Blair doesn't think that urbanization will lead to the extinction of bird species. What it will do, however, is cause the loss of "something nice." "Birders call warblers 'eye candy,'" says Blair. "With urbanization, you lose them and end up with starlings, pigeons, and house sparrows." What to do? Advocate for green space, says Blair, who works with urban planners. In one project, he helped planners in Minnetrista, a community just west of Lake Minnetonka, plan for interconnected green spaces in anticipation of a population surge. Also the National Fish and Wildlife Federation has drawn up guidelines to help homeowners make their yards bird friendly. Of course, we can't raise insects to attract warblers, but means do exist to give those beautiful species that do live in our urban back yards a little respite from the rigors of life in the big city.