Every year, billions of birds migrate north in the spring and south in the fall. The majority of the birds fly at night, navigating using the Moon, stars and night sky. As they pass over big cities, they can become disoriented by bright artificial lights and sky glow. An estimated 600 million birds in the U.S. die each year from collisions with buildings. A study published by Cornell Lab of Ornithology ranked metropolitan areas in Missouri, Kansas and Illinois where, due to a combination of light pollution and geography, birds are at the greatest risk of becoming attracted to and disoriented by lights and crash into buildings.