Migration means when a bird, who can't suffer too much cold and where it's living currently has a very, very cold climate (like snowing, hail , too much rain etc..) and the bird can't tolerate it goes to the other direction (East, West, North and South.) where it is warm....And they come back when it is warm in their hometown ...
A very few birds like Seagulls do the opposite. They can't suffer much heat and goes to another direction where it is cold.
