IN the Middle Ages the disappearance ol the swallows and maytins every autumn, and their reappearance in the spring, was attributed to hibemaUon It was thought that they dived into lakes or ponds and buried themselves in the mud for six months The cuckoo, it was thought, changed itself Into a sparrowhawk and the goose into a barnacle, it was not until the movement of birds began to be studied scienUfically that the truth about long-distance migrations became known though even in classical Uma some migratory flights were recorded.

The swallows and martins go to Africa to winter, and the cuckoos travel even farther south. These birds do not breed in Africa, however, though there are exceptions to every rule. In the same way. the fieldfares and redwings winter in Britain but return to Norway and Russia to breed, while some species of skuas migrate between the Atlantic and Pacific basins

The detailed movements of many birds are now thoroughly well known, since the practice of marking birds (usually be "ringing' them) has been systematically followed for at least fifty years. Nevertheless, two complete mysteries remain to be salved. First, why does any particular species of bird fiy to a distant country at all? Sec ond. how does It know the way?

The answer to the first question is possibly to be sought in the bird's ancestry Ai some remote period a bird s

ancestors may have been native to. say Africa, but circumstances have since arisen which have caused the race to emigrate to. say. Europe For some time the birds might still have an affection for two "homes," which they visit tn turn. It is certainly true thal some birds become strongly attached to their nesUng sites the swallows, for instance, often coming back from Africa to the same nesting-spot year after year, and perhaps using their old nest after a little patching.

As to how the birds find their way, there is no doubt that In the neighbourhood of their journey s end they go by visible landmarks. For the res: of the journey, it is sometimes held that only a very general sense of direction would be needed, for so many birds die on the way that it may be pure chance that any particular bird will be among those to arrive safely

This, however cannot ex plain all lite facts For example it is hard to see how even the general direction can be known to the young cuckoos which leave for Africa a full month before the older birds They are neither shown the way nor can they be taught it by their parents for they are reared in other birds nests' Attempts have been made to find explanations in the earth s magnetism the migrations of the sun north and south of the equator, and other such phe nomena. but so far without even the hint of success Courtesy — Wonder Book of Wonders ■