Answer:
migration, which is a response to an external stimulus
Step-by-step explanation:
The butterflies were migrating in response to external stimuli.
Migration is the movement of organisms from one place to another in response to external stimuli. The stimuli could be for food, shelter, weather, etc.
Flight in response to external stimuli is often due to the presence of danger as stimuli. The adrenal gland releases epinephrine which triggers physiological changes in the body and makes the organism decide whether to run, stay, or fight.
In this case, the butterfly could have been migrating in response to the availability of flower nectars or pollen which serve as food for them. Once flowers are available during the spring, the butterflies come around and when the winter comes and the flowers disappear, they migrate away.