There is an easy answer to this question and there is a more complex answer to this question. Some animals migrate, moving over long distances from south to north or north to south, simply to find warmer weather. In many of these case, the animals travel from cold areas in the northern hemisphere to warmer places where food more readily available. If they stayed in their “home” regions they might not have enough to eat and many would die of starvation and from the cold.
Migration is also crucial to the survival of many species because the adult animals mate and raise their young in warmer climates. The larger numbers then return to their original homes when the weather is warmer, where food is plentiful and the young can grow. Animals often follow a specific route each year as they migrate. For example, many birds fly in large groups along the same path that has been used for centuries.
Animals that feed on grass and other vegetation follow similar paths though they may alter their route to find the best feeding grounds along the way. In fact, some larger mammals change routes significantly depending on plant growth, amount of rain and so on.
Animals that live in the water, such as trout for example, seem to be driven by the instinct to mate, breed and raise the young fish. While these particular fish may spend the biggest part of their lives in smaller streams and rivers, they may move in large schools to large lakes or the ocean to allow the young to grow. Only when the time is right will they return to the place they were born. There are a number of reasons for this – proper food, less chance of being eaten by larger animals etc.
Other animals might move shorter distances to find soil conditions that are right for laying their eggs. Alligators are a good example of this activity. Many of the larger turtles cross miles of water to lay eggs in sand beaches, often returning to the same spot year after year.
The process of preparing for migration, actually making the trip and returning to the original location can take quite a toll on individual animals (and on the population numbers). Animals that live in northern climates must eat well and store up plenty of fat for energy so that they have the stamina to make the long trip. Their bodies are prepared over a period of time. While it may seem they are simply enjoying the warm weather and feeding conditions of their home base, they are also preparing physically for the task ahead.
One great example of the distances covered is the European swallow. Large flocks of these small birds fly from their home bases in Europe, cross the Mediterranean Sea and take up residence on the African continent. They make the return trip again a few months later. In almost every case, migrating animals make these amazing trips so that they can stay warm, eat, mate and survive.