Some days there are no birds other times, there are as many as 200. Willard and his colleagues, including Field Museum co-author Mary Hennen and other Field staff and volunteers, have visited the site every day before sunrise during migration season, sometimes as early as 3:30 in the morning.

'I might not have gone back if I hadn't found anything that first day, and now here we are, 40 years later and 40,000 birds later.' 'I went down early one morning, just out of curiosity, and wandered around and actually found four or five dead birds,' says Willard.

In 1978, Willard, the museum's collections manager emeritus, heard an offhand remark about birds hitting the McCormick Place, North America's largest convention center that happens to be just a mile south of the museum. 'These insights were only possible thanks to over 40 years of work by David Willard at the Field Museum, who led collisions and light monitoring efforts.'

'Our research provides the best evidence yet that migrating birds are attracted to building lights, often causing them to collide with windows and die,' says Benjamin Van Doren, a postdoctoral associate at the Cornell Lab of Ornithology and the paper's lead author.

Download lights out for migrating birds