Why do birds fly south for the winter?

In winter, food become scarce. Insects and nectar, which is found on flowers and which will not be available in the cold weather, would be difficult to find. Survival is every animal's instinct. For this reason, birds flock together to fly south and search for other food source. However, if there is still food available, these birds will not migrate and would stay as long as possible. Some birds live with another food source often stay. Another reason for their migration is the cold. Some birds can't withstand the freezing temperature and need to look for warmer areas.