Unlike larger birds that might climb above the clouds, smaller migrants may try swooping underneath, into altitudes interrupted by the towers. The worst kills happen when a flock, perhaps numbering half a million, flaps toward a lighted tower. Something about the lights draws the birds closer. They begin to circle closer and closer to the tower and its steel tethers. It is unknown what physiological trick is at work.
Many ornithologists believe birds navigate by the stars.
In stormy weather, a tower's lights, intended to warn away airplanes, may mimic the celestial beacons. Larkin used a radar dish several years ago to follow a flock of birds approaching a 1,000-foot tower in Michigan. A substantial number made turns. They were well out from the tower, but the birds were flying in a slow arc. Larkin believes that birds caught in bad weather are afraid to leave a tower's glow.