|
theories based on carefully controlled experiments in aviary-laboratories. A favourite theory with the later nineteenth century ornithologists was that known as the " glacial epoch " theory. According to the more extreme interpretations of this theory, migration arose from the successive advance and retreat of the northern ice-cap, the birds retreating before the ice as it pushed southwards, and returning north again as the ice retracted. In this way, over a succession of ice-ages, birds were said to have evolved a north-south swing which became an inherited faculty. There are many factors which are opposed to this view, not the least of which is the length of time during which the ice-cap covered northern Europe before retreating again. This was a matter of some millions of years, and the retention of an .urge in the birds to return to their original home during so long a period of time is unlikely. It is probable that more can be gained by studying present-day conditions than by invoking prehistoric ones, in attempting to explain the origin of migration. The matter is one of immense complexity and so far as we know, no single factor is likely to offer a discrete or clear-cut explanation. In asking why certain birds migrate, the answer that they do so because, under conditions prevailing in the cold season, the normal food supply is not available, appears on the surface to be adequate. But investigation shows that many species are what we know as " partial migrants " ; that is, while certain members of the species stay in the breeding area all "the year round, others perform long and arduous migrations of a thousand miles or more. It has even been shown that, in certain partial migrants, some of the birds from a single brood may migrate in the autumn, whilst others from the same brood remain at home. The British song-thrush, pied wagtail, and robin are examples of" partial-migrants," and here the food problem cannot explain their migration. Complexities and anomalies thus face the would-be student of migration at every turn. The first thing he must decide on, is a sound definition of migration. Perhaps the best is one which states that " migration is a movement of population in which there is a definite shift in the centre of gravity of the population." In other words the whole or part of a breeding |
