Cumulative Record. Chapter 46: How to Teach Animals. Quote 9

… you can make it appear that a pigeon can be taught to read. You simply use two printed cards bearing the words PECK and DON’T PECK, respectively. By reinforcing responses to PECK and blacking out when the bird pecks DON’T PECK, it is quite easy to train the bird to obey the commands on the cards. (p. 609)