Chicago is part of what is known in orinthological circles as the Mississippi Flyway. Millions of birds pass through our city during the spring and fall migration seasons, but a lot of them get banged up or killed among our skyscrapers. The birds mainly fly at night and the building lights really confuse them.
Thanks to a woman named Robbie Hunsinger, and her organization the Chicago Bird Collision Monitors, a lot of the downtown buildings are now turning out their lights after 11 p.m. to protect the birds. There's more to be done, though. If you live or work downtown, you may want to read further on CBCM's website to learn about simple ways you can help. There's also more in an article I wrote for April's Conscious Choice.