(Inside Science) -- Bats are most active after sundown. That’s when they start to leave their roosts -- caves, rock crevices, old buildings, bridges, mines or trees. It can be a spectacular site when bats come out at night, swarming the sky by the millions. But this nighttime ritual also comes with questions from scientists who study bats -- how do bats use their echolocation to avoid collisions with each other at high speeds, and fly so close together in such high numbers? To find some answers, researchers are sending UAVs up in the air to fly with bats and collect data on their unique behaviors.