Many animals were trained by the ABE staff to play the piano, including rabbits, ducks, pigs, and chickens.  In some cases the tune being played was indiscernible and at other times music would be dubbed over the animal's performance.  In the early days of ABE, chickens were trained to peck at the keys over which a small dot could be moved into place by the trainer.  In this case, the chicken could actually play a discernible tune.

Frequently, the behavior of playing a piano was "chained" to another simple behavior like pulling on a cord that turned on a light.  For example, on cue a duck would enter the stage, turn on a light, and then play the piano for a while before being reinforced.  Other pictures available of the ABE exhibits indicate that cats could also be trained to turn on a light then play the piano.

Photo Gallery (click on each photo to enlarge)