The Cuckoo Sound
When Franz Anton Ketterer devised the concept of the cuckoo clock, he also had to invent a reliable way of simulating the call of the cuckoo.To bring his idea to life Ketterer needed a mechanical system that could recreate the distinctive cuckoo sound; and do this reliably many times a day. He solved this problem using a system of bellows to produce the two tones of the bird's call.
The initial cuckoo clocks simply replaced the chimes of existing wall clocks with the new bird calls. Over time they became more exotic with features being added until the design settled down on the classic carved bird house with a cuckoo automaton popping out of a little door on the hour.
Ketterer's cuckoo clocks quickly took the market by storm. At the height of their production it is estimated that over 13 thousand people in the BLack Forest region of Germany were in some way connected with the cuckoo clock business!
Other Sounds
Nowadays of course modern technology can recreate almost any sound and the "cuckoo" clock doesn't have to house a cuckoo or even a bird. One well known example of this is the MooCoo Clock with a model cow that pops out and goes "Moo"!