Using satellite maps and radar to estimate the number of migratory birds streaming across the night sky, Chicago tops the list of cities where birds face the most danger from light pollution in both spring and fall. North America hosts about 3 billion fewer birds today than in 1970, according to a 2019 analysis published in Science. The causes include light pollution, climate change, vanishing habitat and pesticides. Scientists believe the combination of factors could lead many abundant bird populations toward extinction.

For example, Cornell University ornithologist Andrew Farnsworth found that the seven annual Tribute in Light twin towers anniversary memorials on September 11 that project intense beams of light into the night sky attracted an average of more that 1 million birds. Within the first 20 minutes of each event, up to 16,000 birds crowded into a tight radius. Bird conservationists listen for disoriented chirps and if too many are circling aimlessly in the beams, the lights are turned off.

BirdCast incorporates large-scale weather radar and machine learning to forecast the exact nights when hundreds of millions of migratory birds will arrive over U.S. cities. The team sends the data to conservationists and policymakers to help the birds by dimming lights along the way.