What are Coo Coo Clocks? (correctly spelt Cuckoo Clocks)

A cuckoo clock (commonly misspelt coo coo clock, kuckoo clock, coocoo clock, cockoo clock) is a clock, typically pendulum-driven, that strikes the hours using small bellows and pipes that imitate the call of the Common Cuckoo in addition to striking a wire gong. The mechanism to produce the cuckoo call was installed in almost every kind of cuckoo clock since the middle of the eighteenth century and has remained almost without variation until the present.