Occasionally this results in some hardship, and rarely in the destruc- 

 tion of large numbers of individuals. Cases are known where early 

 migrating bluebirds have been overwhelmed by late winter storms. 

 Nevertheless, unless such climatic conditions are prolonged, no serious 

 effect on the species is noted. The soundness of the bird's instincts is 

 evidenced by the fact that natural catastrophes, great though they may 

 be, do not permanently diminish the avian populations. 



As has been pointed out, the advance of average temperature lines, 

 known as isotherms, is found to correspond closely with the northward 

 movements of certain species. For example, the northward travels of 

 the Canada goose are found to coincide with the advance of the iso- 

 therm of 35° F. (fig. 4). 



The spring flight of migrants, if interrupted for any reason, is 

 resumed when weather conditions again become favorable, and it is 

 probable that all instances of arrival of birds in stormy weather can 

 be explained on the theory that the flight was begun while the weather 

 was auspicious. The state of the weather when a flight starts at any 

 southern point, the relation of that place to the average position of the 

 bird under normal weather conditions on that date, and the average 

 rate of migratory flight, are data basic to any reasonably accurate pre- 

 diction of the time arrival may be expected in northern areas. 



Head winds are as unfavorable to migration as is rain or snow, 

 as they greatly increase the labor of flight and cut down the speed of 

 cross-country travel. If such winds have a particularly high velocity 

 they may force down the weaker travelers, and when this happens over 

 water areas, large numbers of birds are lost. Even strong winds that 

 blow in the direction of aerial travel are unfavorable for the birds, as 

 they interfere with their balance and disarrange their feathers. Mod- 

 erate tail winds and cross or quartering breezes appear to offer the best 

 conditions for the passage of the migrants. 



Problems of Migration 



Banding studies 



The study of living birds by the banding method, whereby great 

 numbers of individuals are marked with numbered aluminum leg 

 rings, has come to be recognized as a most accurate means of orni- 

 thological research. Since 1920, banding work in North America has 

 been under the direction of the Fish and Wildlife Service in coopera- 



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