MIGRATION OF BIRDS. 275 
remarks :* ‘* The Song. Thrush furnishes us with a most 
excellent example of the complex nature of the phenomena of 
bird migration as observed in Great Britain and Ireland, and 
its various movements cover nearly the whole year.”’ 
What is the cause of this widespread migration, and how 
did the habit origmate ? To quote Professor Newton :} ‘‘ Here 
. we are brought face to face with perhaps the greatest 
mystery which the whole animal kingdom presents—a mystery 
which attracted the attention of the earliest writers and can 
in its chief points be no more explained by the modern man 
of science than by the simple-minded savage or the poet or 
prophet of antiquity.” 
The theories are many ; some more or less far fetched, but, 
as Mr. Eagle Clarke{ points out, the question may best be 
answered by asking yet another, namely: ‘‘ What would 
become of those myriads of birds, which in the summer delight 
in and breed amidst the solitude of the Arctic countries, 
when those vast wastes which form their feeding grounds lie 
under a pall of snow or are transformed into solid ice ? What, 
too, would become of certain birds which, similarly, make our 
islands their summer home if they attempted to remain the 
winter with us ?”’ He goes on to point out that insect feeders, 
such as the Swallow and Cuckoo, would perish during the 
English winter for want of food. Bears or various other 
animals solve the problem by hibernating during the winter 
months, and writers from Aristotle down to savants at the 
beginning of last century have held the theory that certain 
birds also hibernated, while the superstition that Swallows 
spent the winter in holes or even under the ice in ponds and 
streams was widely believed in medizval times. 
Modern observation has entirely disposed of the theayy of 
hibernation among birds. Their power of flight enables them 
to escape starvation by moving to warmer climates in search 
of food. The reason for the autumn migration is thus then 
comparatively easy of explanation, but once the birds have 
reached their winter quarters, what is the impulse which calls 
them back in spring ? In some cases the food problem again 


* «‘Studies in Bird Migration,”’ Vol. I., p. 212. 
¢ *‘ Dictionary of Birds,”’ p. 549. 
t * Studies in Bird Migration,”’ Vol. I., p. 15. 
