Besides, in some regions like Europe and Eurasia seasonal climatic and ecological changes occur that may require the birds to move away if they are to survive. This is the most obvious and perhaps the most important advantage provided by migration. By flying hundreds and sometimes thousands of kilometers, birds can trade bitter cold and long nights of northern winters for the gentler warmth and sunlight of southern climates. Conversely, in the summer they can escape the scorching heat of the south and enjoy the long cool days of northlands. The evidence that migration achieves these benefits is found in the avian species of India. Out of 1,301 species, only 41 are endemic. The rest are migratory or seasonal visitors.
Bird migration (Wikipedia)