Notchochord, morphology, chordates
Answer:
This suggests that the moustache is a releaser for aggression.
Explanation:
<em>Fixed action patterns</em> 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 <em>Northern flickers</em> male are an external sensory stimulus. This trait that can provoke another male to respond aggressively, as a f<em>ixed action pattern</em>.
This trait might be considered as an <em>unconditioned stimulus</em>, which <em>provokes an unlearned or reflex reaction</em>. These aggressive responses are triggered by reflex.
