Animals migrate because...
No one knows for sure why animals migrate, but some scientists think it may involve magnetism.
Some scientists have done studies with homing pigeons by putting magnets on their heads. Instead of knowing their way back, the pideons become confused and don't know where they are. Similar studies have been done on spiny lobsters, where when the magnetic field is changed in their tank, they don't know where to go. Some scientists think that they use the earth's magnetic fields to guide them.