A clock not wound runs down.

Seven pigeons were trained to respond on one key during the first half of a 60-s trial, and on another during the second half, to test the prediction that the rate of the internal pacemaker should slow in the absence of reward. The relative probability of responding was well-described by Erlang distributions, which yielded as parameters the period of the pacemaker and the criterial number of counts for switching to the second key. During a subsequent period of extinction the period increased linearly with time in extinction, and the criterial count decreased. This slowing of the pacemaker was predicted by the Behavioral Theory of timing, but not by other theories.