Cuckoo is any one of a group of related birds found throughout most of the world. The name cuckoo comes from the song of the Old World common cuckoo. This song also serves as the basis for the well-known sound of the cuckoo clock. Cuckoos have rather long and slightly curved beaks. They differ from most birds because two of their toes point forward and two backward. In most species of birds, three toes point forward and one toe points backward.