2) Innate route-planners

How do juvenile birds that have never been more than a few miles from the nest before, find their way unassisted to places thousands of miles away to rejoin their parents? The answer, once again, seems to lie in their genes.

In 1958, Perdeck caught over 1100 starlings in the Netherlands from a population that migrates southwest in the autumn. He transported them to Switzerland and found that juvenile birds maintained their southwest course, rather than compensating for their displacement. They eventually reached Spain, an area outside the population's normal range. The young birds were clearly under ‘instructions’ to head southwest.

We now know that more than 20 species of bird will try to fly in the direction of their migratory destination, when caged. These birds hatch with the migration direction specified in their makeup. This has been christened ‘an innate compass direction’. Just how the genes contain instructions to head in a particular direction, remains a mystery.

Inexperienced birds rely on this compass direction on their first migration journey even if they are experimentally displaced.

In at least four species, the genes carry even more specific instructions. Young warblers, captive in Germany, tried to head southwest in September and October, but changed to a southeasterly course in November and December2. The caged birds turned from southwest to southeast just as their wild relatives were doing the same along their autumn migration route. Both captive and wild warblers carry instructions to head southwest in September and to keep flying at a certain speed, for a certain length of time before changing direction southeast. This is called compass navigation.