COLUMBUS — New research finds fresh evidence that urbanization in the United States threatens the populations of some species of migratory birds.
But the six-year study also refutes one of the most widely accepted explanations of why urban areas are so hostile to some kinds of birds.
Most ecologists have assumed that common nest predators in urban areas — such as house cats and raccoons — were destroying eggs or killing young birds in greater numbers than in rural areas, said Amanda Rodewald, co-author of the study and associate professor of wildlife ecology at Ohio State University’s School of Environment and Natural Resources.
But this study was one of the first to actually test that assumption by monitoring natural nests over several years. And the results showed that predators weren’t the main problem: instead, the birds just didn’t seem to like urban areas and gave up more easily.
Urban areas attracted lower-quality birds which, compared to those in rural areas, arrived later in the spring, left earlier in the fall, made fewer nesting attempts and were much less likely to return to nesting spots from year to year.
“There is something about these urban forests that strikes the birds as unsuitable,” Rodewald said.
“Even when they try nesting, they are less likely to renest after failure or to return in subsequent years.”
Rodewald, who also has an appointment with the university’s Ohio Agricultural Research and Development Center, conducted the study with Daniel Shustack, a graduate student in wildlife ecology at Ohio State. Their findings were published in a recent issue of the Journal of Animal Ecology.
The researchers monitored the nesting success in the Columbus area of Acadian flycatchers, a migratory bird that is a relatively common summer resident in wooded areas across much of the eastern United States. It winters in Central and South America.
The study involved six years of monitoring 387 nests and 167 breeding pairs of Acadian flycatchers who lived in 35 forest stands in and around Columbus. The researchers found striking differences in the number of young produced by flycatchers depending on how urban their nesting sites were.
Nonurban sites averaged nearly two young produced each nesting season, while urban nests averaged about one young per year. Nest survival ranged widely across sites, with 11-55 percent of nests successfully raising young.
Nest survival, however, was not related to urbanization. The researchers determined how urban a nest site was by measuring the percentage of land within a 1-kilometer radius that was covered by a building, parking lot, mowed lawn or other manmade surface.
As other research has suggested, this study did find that urban areas had more predators, such as raccoons, when compared to rural areas. But these predators were not raiding nests more often in urban areas, Rodewald said.
So what was reducing the number of young produced? One problem may be that the adults birds who nested in urban areas tended to be slightly smaller — although not greatly so — than those in rural areas.
“The birds are sorting out, and it appears the lower-quality birds are the ones forced into urban areas,” Rodewald said.
“That means they have no other options — there are not better rural areas for them to go.”
The study showed that birds in urban areas started their nests later, already putting them at a disadvantage in the relatively short nesting season.
In rural areas, if a nest failed for some reason early in the season, the flycatchers would often make a second nesting attempt. But in urban areas, the flycatchers would often give up if their first nest in a year wasn’t successful.
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