Barn, masked and bay owls is a family of owls containing 16 species. They are recognised by their distinctive heart-shaped face, which is actually a structure made of feathers and serves the purpose of aiding the owls' hearing. The owls in this family often prey on small birds as well as on the rodents that owls are famed for catching. Their habit of taking the former prey has led to some man-made ecological catastrophes, because they were introduced to islands to control rodents and have instead killed the local birds.