What is the significance of the physiological subcycle of migratory birds?

Some birds are unable to live in certain environmental conditions. For example, during winter, when frost begins and there is not enough food for the birds. Then they are forced to migrate for a while. They have all the possibilities for this, since the birds are adapted for a long flight. But the birds must return, as there is not enough food and space for all the birds in the migration areas. After returning, they begin to multiply, create families. Migration is an important part of the life cycle of many birds.