(CNN) A staggering 600 million birds die every year in the United States after colliding with tall buildings. And Chicago, with its skyscrapers and location on a major migration path, is perhaps the biggest killer.

But for Dave Willard, a collections manager emeritus at the city's Field Museum, the dead birds have been an unexpected scientific windfall. Each morning in the spring and fall, when birds make their epic journey between Canada and Latin America, he has headed out to pick up the dead animals from the street.

"I've just stopped for the season. The number of birds we get each day is highly variable depending on whether it's a big day of migration. The maximum is 300 in a day," he said.

Along with a volunteer group, since 1978 he has collected more than 100,000 dead birds, carefully measuring them with a caliber and scale and cataloging the results by hand in a ledger.

Now, a comprehensive study of the unique and remarkably detailed data Willard amassed has shown that North American migratory bids have been getting smaller over the past four decades, and their wingspan wider. The changes appear to be a response to a warming climate.

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