A cuckoo clock is, typically, a pendulum-regulated clock that strikes the hours with a sound like a common cuckoo's call and has an automated cuckoo bird that moves with each note. Some move their wings and open and close their beaks while leaning forwards, whereas others have only the bird's body leaning forward. The mechanism to produce the cuckoo call has been in use since the middle of the 18th century and has remained almost without variation.