Page:Popular Science Monthly Volume 60.djvu/339

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THE JOURNEYINGS OF BIRDS.

bilities of error. For example, meteorological conditions play an important part during migrations, a rain storm or an unusually cold spell may retard progress for days. Even if the conditions are favorable, it is hardly probable that the same individuals migrate for more than a night or two without intermission, so that while the species may be making progress the individuals are alternating a night or two of travel with often several days of rest and recuperation. Again, it was found that most species traveled considerably faster during the latter part of the journey than during the first part. Thus six species showed an increase of 77 per cent, in speed for the northern half of their journey, and the same general result was obtained by calculating the average speed of twenty-five species separately for each of the different months in which migration is performed; the average for March being 19 miles, for April 23 miles and for May 26 miles a day. The species which are late migrants also move faster than those which start earlier and take more time about it.

The persistence with which birds cling to established lines of travel during the migrations is one of the most remarkable facts within the range of bird life, and this in not a few cases can only be interpreted in the light of past geological conditions. Thus certain species which breed in Europe and spend the winter in Africa now cross the Mediterranean at one of the widest points, a seemingly needless waste of energy. But soundings between these points have shown that the sea for much of the distance is relatively shallow, and that a moderate subsidence has changed what may have been narrowest to what is now one of the broadest points. This subsidence was undoubtedly slow and first resulted in the formation of a series of islands and lagoons, and the birds easily passed from one island to another, and even after the last bit of land had disappeared they still followed the old route established by their remote ancestors.

Many shore and water birds that spend the breeding season in and about the arctic circle to the north of Europe and Asia, follow lines of travel during their migrations that were undoubtedly established under past continental or oceanic conditions. Thus certain species take a circuitous route over what is now a wide expanse of open ocean, while others pass far inland through the Russian and Central European lowlands-Those of the first class are simply still following an ancient shoreline, and those of the second class the location of an inland shallow sea.

The Old World migratory quail (Coturnix coturnix) is one of the comparatively few migrants among the so-called game birds. During the migrations they wander far from places of their birth, reaching South Africa, Persia and India. The individuals inhabiting Great Britain, or at least a part of them, long ago established a migration