Great Slave Lake at ii6 miles a day; and another 5 species cover 150 

 miles a day to reach Alaska. This change is in correlation with a 

 corresponding variation in the isothermal lines, which turn northwest- 

 ward west of the Great Lakes. 



As has been previously indicated, the advance of spring in the north- 

 ern interior is much more rapid than in the Mississippi Valley and on 

 the Gulf coast. In other words, in the North spring comes with a 

 rush and during the height of the migration season in Saskatchewan 

 the temperature in the southern part of the Mackenzie Valley just about 

 equals that in the Lake Superior area, which is 700 miles farther south. 

 Such conditions, coupled with the diagonal course of the birds across 

 this region of fast-moving spring, exert a great influence on migration 

 and are the chief factors in the acceleration of speed of travel. 



Variations in speed of migration in different parts of the country are 

 illustrated also by the movements of the cliff swallow (fig. 3), which 

 breeds from Mexico to Alaska and winters in Brazil and Argentina. 

 It would be expected in spring to appear in the United States first in 

 Florida and Texas then in the southern Rocky Mountain region, and 

 finally on the Pacific coast. As a matter of fact, however, the earliest 

 spring records come from north-central California, where the bird 

 usually is common before the first arrivals are observed in Texas or 

 Florida. The route taken, for many years a migration problem, was 

 solved when it was found that these swallows went around the Gulf of 

 Mexico rather than across it. The isochronal lines on the map show 

 the more rapid advance along the Pacific coast. By March 20, when 

 the vanguard has not quite reached the lower Rio Grande in Texas, 

 the species is already north of San Francisco in California. 



Altitude at which birds travel 



At one time students of bird migration held firmly to the theory that 

 normal migration takes place at heights above 15,000 feet, reasoning 

 (somewhat uncertainly) that flying becomes easier as altitude is gained. 

 Since the development of the airplane, however, and with it man's ex- 

 ploration of the upper regions of the air, it has become common knowl- 

 edge that rarified atmosphere adds greatly to the difficulties of flight. 

 This is due not only to the reduction in oxygen (whether for gasoline 

 engine or the lungs of a bird) but also to the lack of buoyancy of the 

 rarified air. Such birds as vultures, pelicans, cranes, and some of the 

 hawks feel this the least, since compared with body weight the sup- 



26 



