c. How do birds know?

Just a few weeks after hatching, the juveniles of many species manage to find their way to feeding grounds they have never seen before, possibly thousands of miles away, without the help of adults. Cuckoos, for instance, lay their eggs in other bird’s nests to be reared by unwitting foster parents. Every year young cuckoos leave Europe for Central Africa to join other the adults, with no experience or help from their biological or foster parents. How do inexperienced birds know when to leave, which direction to fly in, and when they have arrived? In other words, how is migration controlled?

i. Evidence for genetic control

Some instructions for migration appear to be handed down to migrant chicks in their genes

1) The urge to move

If migrant birds are kept in cages, even if they have never migrated before, they become restless around the time they are due to migrate, hopping from side to side, flying against the bars. Their genes are somehow telling them it is time to go. Such activity was originally called Zugunruhe or migratory restlessness, and has been found in over 150 species. Caged migrants remain restlessness for the length of time it would take to reach the migratory destination in the wild. They therefore appear to inherit a timetable controlling the onset, duration and end of migratory activity.

In 1990 Berthold showed conclusively that the urge to migrate is inherited. He crossbred largely resident blackcaps from Cape Verde with migratory Blackcaps from Europe. The offspring showed some, but not full, migratory behaviour^10,11. This experiment showed that the urge to migrate can be passed on very quickly, even in resident populations.