Migratory birds need city forests

Swainson's Thrush

Swainson's thrush. Image by Minette Layne via Flickr

Chicago’s city motto–urbs in horto–means “city in a garden,” but there’s much more city than garden these days, with urban detritus and suburban sprawl ranging mile upon mile out to the cornfields.

If migratory birds are to survive, a new study suggests, we need more horti in the urbe.

Specifically, we need patches of forest within our cities that will allow migratory birds to forage in some security.

“These findings suggest that remnant forests within urban areas have conservation value for Swainson’s thrushes and, potentially, other migrant landbirds,” said Paul G. Rodewald, an assistant professor of environment and natural resources at The Ohio State University.

“Obviously, larger forest patches are better, but even smaller ones are worth saving.”

Rodewald and post-doctoral researcher Stephen Matthews glued tiny radio transmitters to the back feathers of 91 Swainson’s thrushes they netted in a patch of woods on the OSU campus each spring from 2004 to 2007.

They released the birds in other woody patches throughout the Columbus area and documented how long the birds remained there–testing whether the urban woods provided enough support for the birds before they embarked on the next leg of their migration.

In most cases, the birds remained in each forest patch for about the same time, plumping up before flying off.

“The fact that the stopover duration was similar suggests that all the sites were meeting the needs of the thrushes as they prepared for the next leg of migration,” Matthews said.

The thrushes fled only the two smallest forest patches–1.75 acres and 11.25 acres–suggesting there may be a minimum size for urban woods to sustain Swainson’s thrushes. But more studies are called for, with more species, to determine how urban forests support migrating birds.

“The results are important because, with the expansion of cities worldwide, migrating landbirds increasingly must pass through vast urban areas which offer very little of the forest habitats on which many species rely,” according to the study, published in the current issue of Landscape Ecology.

Swainson’s thrushes are an olive-drab cousin of the American robin. They tend to avoid forest edges, and when not being netted by ornithologists, they migrate surreptitiously.

I’ve only seen them in Chicago after they’ve met their demise. I saw my first at the foot of a Loop skyscraper some years ago. My most recent last week on a sidewalk outside of an auto body shop on Roosevelt Road.

This little olive corpse lay less than a mile from a wild place once known by a few urban advernturers as “The Urban Frontier”–26 blocks of unused railroad land along the Chicago River, just south of the Loop, that once hosted an unlikely forest. The trees were cut down just over four years ago and the land has reverted largely to prairie, though there are–here and there among the weeds and grasses–tender saplings.

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