system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do birds fly south for the winter instead of just staying where its warmer all the time?
gpt
Two reasons: summer days are longer near the poles compared to the tropics, meaning more time for activity; and springtime sees an absolute explosion of food resources (insect and plant population growth) so the birds that fly back north get access to it, whereas the birds that would stay down south continue having to compete for a standard amount of resources.

Basically, seasonal variation makes for some months with an overabundance of food (ie spring) and some months with a dearth of food (ie winter). They fly south to avoid the lack of food.