Northern flickers are woodpeckers of the eastern United States. Males, but not females, have black feathers that resemble a moustache extending from the bill along each side of the head. Whereas aggression between males is common, male and female members of breeding pairs are not aggressive toward one another. However, when scientists painted a moustache on female members of breeding pairs, their mates attacked them. This suggests that - aggression is a learned behavior. - the moustache is a neutral stimulus. - aggression is a conditioned reflex. - the moustache is a releaser for aggression. - the moustache is a conditioned stimulus.

This suggests that the moustache is a releaser for aggression.

Explanation:

Fixed action patterns are instinctive programmed responses in different species that are triggered by a specific external sensory stimulus. These stimuli are called sign stimuli or releaser. When the sign occurs, the animal starts a sequence of acts in response to the stimulus and continuous until the series of actions is completed.

The black feathers resembling a mustache in a Northern flickers male are an external sensory stimulus. This trait that can provoke another male to respond aggressively, as a fixed action pattern.

This trait might be considered as an unconditioned stimulus, which provokes an unlearned or reflex reaction. These aggressive responses are triggered by reflex.