The reasons are complex and not fully understood. But a simple explanation is food and a safe place to breed. Birds which breed in the summer in the extreme north such as the Arctic benefit from an abundance of food as plants and insect life flourish in the long daylight hours; and because few large permanent predators can survive the harsh winter. Many birds that breed in the Arctic simply lay their eggs on the ground. Being able to fly, they can avoid the harsh winter conditions, and be the first to arrive to enjoy the summer benefits.
Migration is affected not only by food supply, but also by wind and oceans currents. These make some routes and locations easier to reach. While many birds migrate from northern breeding areas in the summer, to southern wintering grounds (mainly because there is more land near the northern pole than the southern), there are many other migration patterns. Some birds breed in the far south of South America, Australasia and Africa, and migrate to northern wintering grounds. Some birds migrate horizontally, to enjoy the milder coastal climates in winter. Other birds migrate in terms of altitude; moving higher up a mountain in summer, and wintering on the lowlands.
Records of bird migration were made 3000 years ago by Hesiod, Homer, Herodotus and Aristotle. The Bible also notes migrations, as in the Book of Job (39:26), where the inquiry is made: "Doth the hawk fly by Thy wisdom and stretch her wings toward the south?" The author of Jeremiah (8:7) wrote: "The stork in the heavens knoweth her appointed time; and the turtledove, and the crane, and the swallow, observe the time of their coming."
Aristotle noted that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile. Pliny the Elder, in his Historia Naturalis, repeats Aristotle's observations. Aristotle however suggested that swallows and other birds hibernated. This belief persisted as late as 1878, when Elliott Coues listed the titles of no less than 182 papers dealing with the hibernation of swallows. It was not until early in the nineteenth century that migration as an explanation for the winter disappearance of birds from northern climes was accepted.