Birds do it, bees do it, and flies do it too. However, when flies have sex, they may end up on the menu of a hungry bat. Researchers at the Max Plank Institute for Ornithology observed bats and flies living in a cowshed in Germany. Typically, the flies were nearly invisible to the bats’ sonar at night, but this all changed when flies got busy. During sex, male flies produce noisy broadband buzzing sounds that bats are able to hear and home in on. About five percent of copulating flies were attacked by bats, and most of them were eaten. So now we know why bats swallowed the fly.