Since the distance that some birds travel on migration can be enormous, why such huge distances? Migration is useful for the birds, because the seasonal differences are to the birds' advantage. So far as is known, migration instincts are inherited: the birds cannot make a personal choice in the matter. All the same, in some species not all migrate. There is a cost to migration in the number of birds which die "en route". There is an advantage only if the birds which migrate have a better chance of leaving descendants. Studies of migration have been done since the 18th century. There is still much we do not know about it.