Owls are natural predators that do most of their hooting and screeching at night because they are nocturnal animals. This means they do their hunting and perform other activities primarily during the nighttime hours.
Owls hoot for a number of reasons, but just like any other bird or animal, they use these sounds to communicate. For example, great horned owls stick together for life and will hoot back and forth to remind each other of the permanent bond they hold. Other owls, such as the barn owl, will use different hoots to communicate back and forth with one another or to locate another nearby owl. Female owls often start first by hooting a few times before a male owl responds with a lower pitched hoot. Some owls actually hoot for a half hour or so before finally going to sleep.