Russell Kasper, age 12, of Ottawa, Ontario, Canada, for his question:

Why do birds migrate?

What we know about migration has been learned since bird banding became common early in this century. The task of tracing thousands of species on their seasonal tours is enormous, and still far from finished. We now know that migration has several advantages and a few disadvantages. But at present, no one can pinpoint one single reason why the fantastic migration of birds is absolutely necessary.

The appearance and disappearance of the birds has baffled mankind for ages. And from time to time, naturalists have set forth some outlandish reasons to explain it. For example, the great Aristotle of ancient Greece thought that storks went into hiding and that, come spring, the robins in his neighborhood transformed themselves into redstarts. However, he allowed that pelicans fly south for the winter.

In the 1700s, opinions were even more confused. One naturalist suggested that birds fly to the moon. On arrival; after 60 days, they find no food and hibernate until time comes to fly home again. Other experts of the 1700s thought that birds hibernate. Martins were supposed to hide under roofs and swallows in the mud.

This guesswork seems fantastic, but the truth itself, is amazing. Come fall, a third of the world's birds fly hundreds or thousands of miles to winter in milder climates. Come spring, they fly all the way back to their ancestral nesting grounds. Each species travels its favorite routes between its favorite winter and summer homes.

We now know where most species migrate and also many of their fly¬ways. We know that they time their flights and chart their routes from angles of sun's rays. We know that there are advantages and disadvantages to migration. But at present, no one can say that birds migrate for just one particular reason. There may be several reasons or there maybe an all important reason that nobody has suspected.

It seems logical that they depart to escape the winter. But birds are sturdy little creatures, well able to withstand the cold and wet. In fact, nearly half of our North American birds stay around through the winter. However, winter also brings food shortages, especially insect food. Obviously the insect eating birds have good reason to migrate to warmer climates where their favorite food is available all winter. But many seed eaters also migrate, ignoring our plentiful weed seeds.

Another mystery is why some species migrate short hops, some cross the Equator and a few migrate between the north and south polar regions. Still another mystery is why birds who migrate to friendly southern regions bother to come back in the spring.

Bird migration is an enormous project and every year countless little travelers perish in storms and fogs. Obviously some species need to migrate to find winter food. But for many species, the hazardous journeys seem hardly worthwhile. It is just possible that the main reason for migration is still a well kept secret of the bird world.