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With a neon-green net in hand, Annette Prince briskly walks a downtown Chicago plaza at dawn, looking left and right as she goes.

It's not long before she spots a tiny yellow bird sitting on the concrete. It doesn't fly away, and she quickly nets the bird, gently places it inside a paper bag and labels the bag with the date, time and place.

"This is a Nashville warbler," said Prince, director of the Chicago Bird Collision Monitors, noting that the bird must have flown into a glass window pane of an adjacent building. "He must only weigh about two pennies. He's squinting his eyes because his head hurts."

For rescue groups like the Chicago Bird Collision Monitors, this scene plays out hundreds of times each spring and fall after migrating birds fly into homes, small buildings and sometimes Chicago's skyscrapers and other hulking buildings.

A stark sign of the risks came last fall, when 1,000 migrating birds died on a single night after flying into the glass exterior of the city's lakefront convention center, McCormick Place. This fall, the facility unveiled new bird-safe window film on one of its glass buildings along the Lake Michigan shore.

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