dependent on proper wind conditions for migration. In the fall, often 

 the best day to observe hawk migration in the eastern United States 

 is on the second day after a cold front has passed providing there are 

 steady northwest to west winds and a sunny day for production of 

 thermals (Pettingill 1962). Considerable drifting may be observed in 

 this group of birds because they are literally carried along by the 

 wind or glide from one thermal to the next. Haugh and Cade (1966) 

 found most hawks migrated around Lake Ontario when winds were 

 10 to 25 miles per hour, but, if the wind exceeded 35 miles per hour, 

 most hawk migration stopped. 



In conclusion then, we can say that the weather may be the impetus 

 for migration for many species, but it cannot stimulate a bird to 

 migrate unless it is physiologically prepared. Arrivals on the ground 

 are not necessarily indicative of the number of birds passing 

 overhead. During the fall, peak migrations usually follow the 

 passage of a cold front when the temperature is falling, the 

 barometer is rising, winds are from the west or northwest, and the 

 sky is clearing. In the spring, most migrants proceed north in the 

 warm sector of a low when winds are southerly, warm, and moist, but 

 rain, fog, or snow will often curtail the passage of migrants or 

 prevent the initiation of a migration. Evolution of migratory 

 behavior has probably resulted from the survival of birds capable of 

 selecting those wind conditions, which reduce flight time and energy 

 consumption, during their passage. 



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