Because there are seasons. These result in temporary differences in food availability, weather, light conditions, etc. Migratory animals make use of such differences and exploit resources which are available in different parts of the globe only during part of the year or of their life cycles. They have evolved so that they can live for some time in areas that could not sustain them the whole year round. Migration is a natural phenomenon by which individuals of given species move between areas which they inhabit at different times of the year.
Migratory movements tend to be regular and largely predictable.
In this latter case, many species use regular staging areas as stop-over sites where they recover from the previous leg and gather fuel before they continue their trip. Migratory species of animals are, on average, more at risk of becoming endangered than non-migratory species. As a result, many migratory species that were once common are becoming increasingly rare. We all share a responsibility in the conservation of this common natural heritage. Some species lead such secretive lives that not enough can always be done to improve their conservation status.