76 Wild Birds and thier Haunts 



THE AUTUMN MIGRANTS. 



PROFESSOR J. A. Thomson, who in his recently- 

 published book, ' ' The Biology of the Seasons/' 

 has collected a number of data and calculations 

 which have been made as to the questions of flight and 

 pace, devotes several very interesting pages of his book 

 to enlarging on the theory that the mainspring of the 

 phenomenon of bird migration belongs to the Ice Age. 

 If (adds the Spectator) we may accept the principle of a 

 migratory instinct, he asks, what are the conditions which 

 led to the establishment of this instinct ? He imagines 

 a gradual change of conditions of climate which in turn 

 evoked a changed type of bird. 



There was a time when Greenland and other northern 

 countries had as mild a climate as Penzance. Gradually 

 the climate of the whole northern hemisphere changed : 

 the snow-line came lower on the mountains, great glaciers 

 formed, and birds moved further and further south after 

 food. After a time it became impossible to breed in the 

 accustomed places ; and so there arose a new and re- 

 volutionary type of bird to meet these changed con- 

 ditions, a bird " who would not take hard times lying 

 down, who was sensitive, alert, restless, unconventional, 

 adventurous, and original, who was a genius, in short a 

 Columbus-bird." And the less alert, less adventurous 

 birds who would not journey forth with the Columbus- 

 birds were in process of time eliminated. All the birds 

 who survived had a strong sense of direction and of the 

 necessity of changing ground, and this sense of direction 

 became confirmed into an abiding migratory instinct. 



Then, in a later age, when the Ice Zone had retreated 

 again, and the northern hemisphere had become once more 

 habitable, there came back to these adventurous birds, 

 with their strong sense of direction, ' ' an organic reminis- 

 cence of the original headquarters before the Ice Age." 

 And so, Professor Thomson argues, the northern migra- 

 tion became an established habit in spring, and the 



