Answer:
This suggests that the moustache is a releaser for aggression.
Step-by-step 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.