The Brelands used operant conditioning concepts learned from B. F. Skinner to train animals to perform. However, the animals often demonstrated , which is the tendency for drive to undermine conditioned behaviors.

The animals demonstrated instinctive drift.

Explanation:

Keller and Marian Breland were former students of B. F. Skinner in the University of Minnesota. From Skinner, they learned about operant conditioning, the possibility of teaching desired behaviors to anyone via reinforcement. However, while conducting their own studies, the Brelands discovered what they called instinctive drift. The chickens whose behavior they were conditioning had a drive to perform an unplanned scratching and pecking behavior, with the intention of satisfying their feeding needs. Something similar, also related to food, happened while they were training raccoons and pigs.