Final answer:
Birds fly south in the fall due to both proximate and ultimate causes. The immediate triggers for migration are changes in daylight hours and temperature, which classify it as proximate causation. The ultimate cause of this behavior is related to higher reproductive success in subsequent seasons.
Step-by-step explanation:
Bird Migration
In the case of birds flying south in the fall, it is an example of proximate causation. This means that the behavior is influenced by immediate causes or triggers, such as changes in daylight hours and temperature. The underlying ultimate cause, on the other hand, is related to reproductive success. Birds that fly south in the fall have higher reproductive success in the following spring than those that do not migrate.
This pattern has been observed in the past and is passed down through generations. Therefore, the behavior of birds migrating south is a result of natural selection favoring the individuals that engage in this behavior.