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OSU study finds birds reproduce better in warm, urban areas
KEITH ARNOLD
Special to the Legal News
Published: December 3, 2020
When Ohio State University researchers wanted to evaluate the effects of growing cities on central Ohio wildlife, they set their sights on tree swallows a species which has benefited from urban dwelling with some risks.
In a study supported by the National Science Foundation, the Ohio Department of Natural Resources and the Ohio Water Development Authority, researchers found that city living appears to improve reproductive success of the migratory birds when compared to breeding in more environmentally protected areas.
The birds bred more nestlings because of warmer local temperatures, scientists found.
Specifically, egg-laying occurred significantly earlier by nearly eight days. Also, the clutch sizes were larger and the number of fledglings that left the nest was higher at the urban sites than at protected sites.
The reproductive success was largely attributable to the temperature: The air was warmer in urban sites than in protected sites by an average of 3 degrees Fahrenheit, and city locations had fewer extremely cold days, the study found.
“We were looking at urbanization in two different ways as a category of urban versus not urban, but also as gradients of urbanization,” said the study’s lead author Mažeika Sullivan, director of the Schiermeier Olentangy River Wetland Research Park at Ohio State. “As the amount of urbanization increased, fledgling success increased.
“This tells us local climate is extremely important for reproductive success of tree swallows, and likely other insectivorous birds.”
The success, however, came with potential health risks related to poorer water quality in urban areas, researchers noted.
Insects that emerge from water constituted roughly a third of the tree swallows’ diet and those insects also tend to provide more nutrition and energy than terrestrial flying insects.
That resulted in much higher levels of mercury in the birds’ blood 482 percent higher in city-dwellers than in those who bred in protected sites, the study yielded.
The nation’s total amount of urban land has grown the past three-quarters of a century nearly by the same amount, a 470 percent increase.
“With urbanization expanding worldwide, we are transforming the landscape,” Sullivan said. “And this isn’t going away.
“My lab is looking at how urbanization affects multiple responses of ecosystems what those changes are and quantifying them, but also seeing what this tells us about how we can manage and conserve ecosystems and wildlife in this context. Our task, knowing wildlife are using urban settings, is to think about ways to maximize benefits and minimize the risks. There’s nature in cities. So how can we make our cities a little more wild?”
From 2014 through 2018, researchers observed tree swallows during their breeding season, most of May and June each year, also measuring their body weight and blood glucose and mercury levels and tracking how much of the birds’ food source came from insects emerging from water or starting their pre-winged life on land.
Scientists also monitored temperature and chemical water quality and quantified percentages of forested or wetland versus developed land at the breeding sites.
“This is an important warning,” Sullivan said. “There’s a whole suite of environmental contaminants out there pesticides, a lot of other heavy metals. So, despite the advantage for breeding in these urban areas, there can be a trade-off in individual health.”
These findings, and the implications for birds and other wildlife affected by growing urbanization, suggest that productive urban habitats should be factored in to future municipality planning, he said.
Some top considerations for insectivorous birds, as Sullivan sees it, would be establishing protected green ways, creating nesting habitats and structures to replace lost trees and other parts of the natural landscape, lowering the chances for contaminants to stream into waterways and protecting wildlife food sources by reducing the use of pesticides and insecticides.
“If we had found that urbanization was negatively affecting tree swallows in all the different measures we used, that would be a very different story,” Sullivan said. “But I see some rays of hope here.”
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