herring gull chicks peck at a red spot on their parents bill that causes the parent to regurgitate their food to feed to the chick. tinbergen and perdeck tested the responses of herring gull chicks to cardboard cutouts of adult herring gull heads. they found that chicks pecked nearly as much at a model of just a bill with a red spot as they did with a model of an entire head with red spot. they pecked much less at the model of the complete head without the red spot. what purpose does the red spot serve in this behavior?