Answer:
C. Birds moving south when the weather gets colder
Step-by-step explanation:
The seasonal changes are the changes, which occurs in accordance with the changes in the season. The seasonal migration in birds is a good example of seasonal changes, in this, the birds move from one place to another to stay away from harsh weather and come back again when the weather is suitable.
Other changes in the given questions are terminal changes and lead to the damage to ecosystem. These changes are not the seasonal one.