DISTANCES TRAVELLED BY BIRDS 79 



their actual route is uncertain, but they have been 

 met with in Amazonia, and are known to winter 

 in Argentina, and, it is suspected, in eastern 

 Patagonia. 



The return migration is, so far as it is known, in 

 a steady northerly direction, rather north-west 

 across Bolivia towards Central America. From 

 Yucatan they cross the Gulf to Texas, then slowly 

 travel up the great Mississippi highway and across 

 Canada to their northern breeding grounds. ; ' Its 

 round trip has taken the form of an enormous 

 ellipse with a minor axis of 2000 miles and a major 

 axis stretching 8000 miles from Arctic America to 

 Argentina." 



The following is Mr Cooke's suggestion of the 

 origin of this great ellipse. Towards the close of 

 the glacial era, when the ice began to recede, the 

 Florida peninsula was submerged and only a small 

 area in the south-east of the States was free from 

 ice. Plover attempting to follow up the retreating 

 ice were confined to an all-land route from Central 

 America through Mexico to the western part of the 

 Mississippi Valley. As the east gradually became 

 uncovered the route would be extended to the north- 

 east, until the area stretching to the Great Lakes 

 was fit for bird-habitation. As the route lengthened 

 and the power of flight developed, there would be 

 a tendency to shorten the line by cutting off some 



