Why do birds migrate?

Seasonal migrations of birds are annual flights of birds for considerable distances from their nesting place to the wintering place and back. One of the main reasons for migration is the reduction or even complete disappearance of the food supply in winter. As a result, birds are forced to migrate to areas with milder climatic conditions, where it is possible to find food and not die because of hunger and cold. First of all, this applies to birds which feed on insects of certain species, small rodents or cold-blooded animals, such as frogs. Birds begin to prepare for migrations beforehand. Some of them start migrations earlier than other kinds. It depends on weather. Some birds can delay, if the weather is not so cold. Interesting to look at the birds, flying high in the sky and crying something in their own languages. The most wonderful view is cranes' migration, I guess. They even can make different nice figures in the sky and tell us about their flying.