"Supernormality" in a learning situation

Four pigeons were trained to peck at a 37-spot stimulus for food. While this training continued, two of the pigeons received extinction training to a 21-spot stimulus and the other two received extinction training to a plain white stimulus. In a subsequent generalization test of stimuli which varied in number of spots, pigeons extinguished on the 21-spot stimulus responded most to the 52- and 69-spot stimuli. This "peak shift" in responding is functionally similar to "supernormality" seen in the egg-retrieval responses of gulls. Possible causes of these two phenomena are discussed and compared.