(CNN) A staggering 600 million birds die every year in the United States after colliding with tall buildings. But for Dave Willard, a collections manager emeritus at the city's Field Museum, the dead birds have been an unexpected scientific windfall. The number of birds we get each day is highly variable depending on whether it's a big day of migration. 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.

Source: CNN December 04, 2019 13:52 UTC