### Instruction:
Why do birds migrate north to south for the colder months in the north?

### Input:
Bird migration is the regular seasonal movement, often north and south, along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by humans, and is driven primarily by the availability of food. It occurs mainly in the northern hemisphere, where birds are funneled onto specific routes by natural barriers such as the Mediterranean Sea or the Caribbean Sea.

### Response:
Birds primarily migrate due to the availability of food in warmer climates. The north is a breeding ground, while the south is their wintering ground.