Operant chambers have at least one operandum (or "manipulandum"), and often two or more, that can automatically detect the occurrence of a behavioral response or action. Typical operanda for primates and rats are response levers; if the subject presses the lever, the opposite end moves and closes a switch that is monitored by a computer or other programmed device. Typical operanda for pigeons and other birds are response keys with a switch that closes if the bird pecks at the key with sufficient force. The other minimal requirement of a conditioning chamber is that it has a means of delivering a primary reinforcer (a reward, such as food, etc.) or unconditioned stimulus like food (usually pellets) or water. It can also register the delivery of a conditioned reinforcer, such as an LED signal (see Jackson & Hackenberg 1996 in the "Journal of the Experimental Analysis of Behavior" for example) as a "token".