refer to the following passage
Birds used to travel from one place to another. They travel to the southern warmer lands from northern regions of Asia, Europe and America during the autumn and early winter. During the spring and early summer, they return as they do not get delay till the bad weather. During their travelling they face sufferings through the air, over the hill, forests and plains, sudden storms drive them far out of course, Bright lights at night attract and confuse them as well. They cannot fly as fast as they could. Their migration speed is from 48 km an hour and can exceed 80 too.

Why do the birds travel to the southern region?