Behavioral psychologist tries to train a bird to climb a tree to get a reward of a piece of fruit. At first, the bird learns how to climb the tree with its legs and beak. After a while, it starts flapping its wings and hopping around before it starts to climb. Eventually, the bird flies up to the piece of fruit, even though that prevents it from getting the fruit. According to the Brelands' analysis of biological constraints, the bird is demonstrating .

b. that it was reverting to behavior that was instinctual for it.  

Explanation:

When observing the animals, Keller and Marian Breland saw how sometimes they learn some behaviors ore easily than the other times. They noticed how animals will likely revert to instinctual behavior rather than learned one when they are trying to supply themselves with food. Even when one pattern of behavior is learned for years, once an animal is under the primal need of food, they are returning to their instinctive behavior.

They called this phenomenon as instinctive drift or instinctual drift and the pair called it "a clear and utter failure of conditioning theory."

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