How does migration help organisms survive in extreme conditions?

They can't
Geese in Northern Canada can live there in the summer. As it gets cold, they move south to more hospitable weather. But it's a losing action! With many more geese, there would be mass extermination. Ah, but the geese in southern Canada migrate (invade) Northern USA. The pattern continues as if all the geese were part of one hand that moves its small finger out of the cold one month and then moves its thumb out of the heat months later. The whole hand moves just to accommodate the small finger and then the thumb. So it is with geese. This forms a model for other species that migrate.