Why do birds fly south?

Do you ever question animals intelligence? I think animals, especially birds are smarter than we think. For example. How do birds know when to fly down south for the winter?
Maybe they have this inborn sense that tells them when to fly away. Maybe they can tell by the sun. The suns position and how it gets darker as we get closer to wither. Or it could be as simple as the changing weather.
There are maney options to how birds know when to migrate down south. Its just being able to figure out the write one. If we could ony understand bird language. We would know the answer.
Birds are an interesting aninamal to study. I would love to find out why it is they know when to migrate down south?