Many species of gulls feed their chicks by regurgitating food. When parents return from foraging, the gull chicks peck at a red spot on the parent's bill, which causes the parents to open the bill and regurgitate their food. Chicks will peck at a red dot painted on any long, slender object. For the chicks, pecking is a and the red dot is a . For the adult gulls, the chicks pecking at their bills is a and regurgitating food is a .

fixed action pattern, key stimulus; key stimulus, fixed action pattern

Explanation:

An innate behavior which specific to a species and is highly stereotyped is termed as a fixed action pattern.This behavior occurs due to a neuronal network that is hard-wired in the species and the response is said to be generated following a key stimulus. A key stimulus is the one that generates a fixed action pattern and this is also called a releaser. A key stimulus can be in the form of shape, sound, color, or a combination of these. In this case, the red spot is a stimulus for the chicks which causes them to peck at their parent's bill, and hence, the red spot is a key stimulus, and pecking of the chicks is a fixed action pattern. The parents regurgitate the food only when the chicks peck at their bills, so pecking is a key stimulus for the parents which causes them to regurgitate the food which is a fixed action pattern.