"Migration" in this passage means

M: Every year, as the seasons change, many birds, fish, insects and land animals start to move. Some travel thousands of kilometers. For many it is for the first time. All of them have one thing in common: they know where they are going and they make no mistakes.
     This is migration. There are many things which help animals to migrate. Birds and other animals have a body clock which tells them about changes in temperature and periods of light. Birds and some insects use their eyes to measure and compare the position of the sun from different place.
     Many people now believe magnetism is very important. The earth itself is a magnet and every kind of life is affected in some way by its magnetic, fields.
     But there are still many questions without answers, for example, the migration of eels. European eels travel down their rivers and into the Atlantic Ocean. They then swim 5600 kilometers to the Sargasso Sea, where they lay their eggs. The eels then die. The baby eels are carried by the Gulf Stream to Europe, a journey which takes two and a half years. They swim back up the rivers. There they live for about ten years before the return journey to the Atlantic. How do they do this? We know what happens to the eels but not how or why.