AUTUMN MIGRANTS.
[To THE EDITOR OP THE "SPECTATOR."] SIR, —I was much interested in your article on "Autumn Migrants " in the issue of September 23rd. May I, however, offer an alternative theory to Professor Thomson's ? It seems to me a rather airy hypothesis which assumes that the "organic reminiscence of the original headquarters" returned conveniently after the Ice Age was over. Such a reminiscence, if it existed, would have cropped up continually during the intermediate generations, and the individuals who gave way to the impulse would have been promptly eliminated by " natural selection," their old breeding place being still ice- bound. My own theory occurred to me through watching martins feed their young in June from 3 a.m to 9 p.m., i.e., for eighteen hours in the day. Now if they feed their young for eighteen hours, the latter presumably require it ; but in a southern country, where the day is only twelve hours long, they would perforce have to fast for twelve hours. I take it that migrant birds originally lived in one place throughout the year, but those who went a little further north to breed reared stronger nestlings owing to the longer sum- mer day, and so had a slight advantage in the struggle for existence. Of these the individuals who moved south in winter when insect food became scarce gained a further advan- tage, so that gradually the non-migrants at either end were eliminated. Moreover, the further the migration was pushed within certain limits the greater was the advantage. This theory has at least the merit of assuming a gradual and uniform development of the migratory instinct in place of the sudden revulsion postulated by Professor Thomson. Furthermore, I do not see how the "organic reminiscence," which took the professor's birds back to their "original headquarters before the Ice Age," came to act in the reverse direction with the approach of autumn. As regards the height at which the migrants fly, is it not probable that, like balloonists, they seek the level at which there is a strong current blowing in the desired direction ? This would also account for the rapidity
Previous page
