Wallace R F, Osborne S, Norborg J, Fantino E
Science. 1973 Dec 7;182(4116):1038-9. doi: 10.1126/science.182.4116.1038.
The presence or absence of a change in the ambient stimulus conditions upon entry into a food source controlled the frequency with which pigeons choose one of two concurrently available grain sources. Such changes characteristically accompany the production of response-produced food and account for prior reports of responding to produce food in the presence of freely available food.
