THE UNCHARTED SKY 139 



** That's a mighty hard question to answer," 

 the expert replied. "I don't think that I ought 

 to give you the idea that we know, because there's 

 a good deal of discussion about it among orni- 

 thologists. Most of us are agreed that the migra- 

 tion of birds is connected with the great changes 

 of climate that have occurred during the millions 

 of years that have elapsed since birds first began 

 their journeys in North America. 



''In what were known as the Ages of Horror, 

 or the Great Ice Period, and which lasted at least 

 a quarter of a million years in its four stages, the 

 permanent ice-cap in North America covered all 

 Canada and the United States as far as a line run- 

 ning from New Jersey through Missouri. Un- 

 doubtedly, the winter climate was extraordinarily 

 severe over the whole of the rest of the United 

 States, even as far south as Florida. 



''Birds which had nested and lived in North 

 America prior to the Ice Age, were gradually 

 pushed southward. There was no food in the 

 north, and the struggle for existence must have 

 become fearful in the limited part of the earth 

 which was free of ice. As the ice retreated, it 

 gave more area in which the birds could spread. 

 As it advanced anew, it restricted the area. The 



