WHEN all is known that it is possible to know about birds, they still remain the most elusive of all creatures, close as they are about us in our daily life. ' They have their ways, we ours.' And of all their ways the most enigmatic to the human mind is their habit of migrating-or, in the ease of starlings, of ' hosting' to migrate and never achieving it. Whence, for instance, comes the strength, in such morsels of life, to cross wide seas What guides them ? What urge do they receive towards this sudden gregariousness ? Ono could go on with such questions, many of which still remain, for all the efforts of omi. thologists, but uncertainly answered. Professor J. Arthur Thomson , who is giving this talk from Aberdeen, is one of the most popular writers on biology, zoology, etc, of our day. He is Regius Professor of Natural History at Aberdeen University.