Birds migrate because they need somewhere to breed. At first sight, this statement might seem rather strange. Surely they migrate to avoid adverse conditions during the part of the year when they aren't breeding? To understand why it makes sense, we first have to understand why birds choose to breed in certain places and not in others.
Birds of all species need a few essential resources:
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Somewhere to build a nest:
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If a species happens to arrive by dispersal at a site that provides it with all of these requirements, and enough offspring are produced each year to replace those that die, the species will establish a population at the site.
For most species of birds, breeding only takes place during part of the year. For the rest of the time the birds breeding at a site have a choice. Either they can stay where they are, or they can go somewhere else. They choose the option that makes it most likely they will be around to breed the following year.
Suppose the blue line in the diagram below represents the edge of the breeding range of a species, and the white dots are individuals that stay put, while the orange dots are birds that move somewhere else at the end of the breeding season. If the habit of moving suddenly arises in a population, perhaps because a bird arrives at the site from elsewhere, and the birds that move survive better outside the breeding season, the moving birds will become an ever larger proportion of the population until the habit of staying put disappears altogether.
So the fact that a bird migrates doesn't necessarily mean it can't survive over winter; just that the choice of staying put isn't the best one for that particular breeding site. If a bird wants to breed there, it has to migrate, and this is why birds migrate in order to get somewhere to breed.
In the scenario set out above, the migrants simply parachuted in to the population, so this tells us nothing about how migration arose in the first place. To understand this we need to approach the problem from another angle.