Because the weather in the North of the world have a very cold winter. Migratory birds are living in the country to the South where the warmer and they will return home when flying back into the rain.
Because the weather in the north of the world is very cold. Birds are immigrants living in the warmer south. And they had to fly back when the rainy season.
Because the air in the northern part of the world have a cold. The birds will migrate to living in the country in the South with warm weather. And they will fly back into place when the rainy season.