Why do birds migrate?

Not all birds that migrate it would die during the winter, many of them could survive the winter without any problem, it is imperative in this regard the need for migration.
However, the instinct of migration of migratory birds during the winter is hidden in your DNA and has been handed down from generation to generation since the Ice Age and postglacial.
Instinct and migratory behavior is triggered at a certain time of the year due to the segregation of a hormone, even caged migratory birds have a nervous defendant during this time of year.

There are sedentary birds, such as gallinaceous, sparrows, tree creepers ... perform small migrations birds of passage as suckers, write, finches and others.

Outside the breeding seasons together in large flocks made not too long territorial movements.

Then we have birds, partially migratory, as blackbirds, robins ... where some individuals migrate while others stay in the breeding areas.

Finally the long-distance migratory birds like swallows, swifts, shrikes, flycatchers and robins will make a long journey to Africa, where they await the winter and return in mid-April or early May.