Between 1500 and 4000 species of birds migrate, the real answer is not really know yet and the experts differ as to their answers. Some birds travel south in some winters but not others. This is not migration, but dispersal resulting from a population boom. These population booms can occur regularly, i.e. every 4 or 10 years perhaps, but there is no return in the spring. Destruction or loss of food resources can also cause the irregular eruptions of birds to new habitats.
Seasonal migration is a major factor in the life of many birds.
In some cases this migration is very obvious and involves huge distances but at other times it is much more subtle. In the UK most people know that Swallows, Swifts and many small passerines such as Wood Warblers and Redstarts are migratory birds. They are present in the summer, but fly south along with many other species to warmer climes for winter. Similarly, many ducks and geese are only present in the UK during winter, when we are their south. They return north to the nearctic in summer to breed.