Many office towers leave the lights on at night, a wasteful practice that has lethal consequences for migrating birds. Confused by the lit windows, millions of birds slam into tall buildings and die every year, says Doug Stotz, a conservation ecologist at the Chicago Field Museum. He and his colleagues quantified the losses by counting the number of birds that hit McCormick Place, a 90-foot-tall glass convention center in Chicago. Over the last two years, 1,297 birds—mostly sparrows, warblers, and thrushes—perished after flying into illuminated windows. During the same period, only 192 died from hitting dark windows, even though the windows were unlit nearly half the time during the study. The deaths place an additional strain on declining species such as Wood Thrushes. "This is just another pressure that the birds don't need," says Stotz. Because most window collisions occur during the peak migratory times after midnight, he suggests that skyscrapers turn out their lights from 11 p.m. until dawn during the migration season, from late March through May and from mid-August to Thanksgiving. A seasonal lights-out campaign would also reduce the buildings' electricity bills.