Why do birds migrate in the winter?

I’ve always asked myself that… Why all birds except for a couple smaller ones migrate South in the Winter? Are they actually cold or is there something else taht’s going on in the weather?

I’ve been researching this for over 4 hours and came across this amazing video, check it out below, it will blow your mind.

Isn’t this amazing, I couldn’t believe my eyes when I first realized where they actually go.