xviii BIRDS OF AMERICA 



uneconomical way of applying power, since all the force required to bring the wing forward 

 for the beginning of the stroke is not only wasted, but more than wasted, as it largely increases 

 the air friction and retards the speed. On the other hand, the screw propeller of the aero- 

 plane has no lost motion. Yet less than 2 ounces of fuel in the shape of body fat suffice 

 to force the bird at a high rate of speed over that 2,400-mile course. A thousand-pound 

 aeroplane, if as economical of fuel, would consume in a 20-mile flight not the gallon of 

 gasoline required by the best machines but only a single pint. 



The Canada Goose is typical of what may be called regular migration. This bird 

 fulfills the popular notion of bird migration, /. c, it moves northward in spring as soon as 



ISOTHERM or 35° F 

 ISOCHRONAL MIGRATION LINES 



Tnurtesy of U. S. Dept. of Agriculture 

 MIGRATION OF THE CANADA GOOSE 

 An example of migration keeping pace with the advance of spring 



the loosening of winter's fetters offers open water and a possibility of food. It continues 

 its progress at the same rate as spring, appearing at its most northern breeding grounds 

 at the earliest possible moment. The isotherm of 35° F. seems to be the governing factor 

 in the rate of spring migration of the Canada Goose and the isotherm and the vanguard 

 of the Geese are close traveling companions throughout the entire route. Moreover, the 

 isochronal lines representing the position of the van at various times are approximately 

 east-and-west lines during the whole migration period. But this so-called regular migration 

 is performed by a very small percentage of species, the great majority choosing exactly 

 the opposite course — to remain in their winter homes until spring is far advanced and then 

 reach their breeding grounds by a migration much more rapid than the northward advance 

 of the season. 



Much has been learned about bird migration in these latter days, but much yet remains 

 to be learned. 



