INFLUENCE OF WEATHER 



It is thought by some that the weather has little to do with the time 

 of arrival of migratory birds. It is assumed that if the bird is 

 physiologically prepared for migration it departs, irrespective of the 

 weather. Even if this were the case, weather can influence the 

 progress of migration by not only controlling the advance of the 

 seasons but also by helping, hindering, or even stopping bird flight 

 (Welty 1962). 



Some scientists believe that birds not only avoid bad weather at the 

 start of a journey but usually finish the journey in good weather 

 (Nesbit and Drury 1967b). Contrary to what many observers believe, 

 the arrival of birds in an area, whether they stop or continue on, is 

 more often controlled by the weather at the point of departure than at 

 the point of arrival. During the peak of migration, suitable weather 

 may occur at an observation site, but strong migratory movements 

 may be arrested before the birds arrive there because the weather 

 was not suitable at the point of departure or somewhere in between. 

 In addition, if there is good weather at the point of departure as well 

 as farther down the migration route, the migrants, once air-borne in 

 a favorable weather pattern, may continue on right over an expectant 

 observer and the whole flight will be missed. Nesbet and Drury's 

 (1967b) radar study on air-gound comparisons found, with few 

 exceptions, ground observers missed the largest movements observed 

 on radar. Observation of a large wave of arrivals indicated migrants 

 had been stopped by a meteorological barrier, and people were 

 actually not reporting maximum migration but an interruption to 

 migration. Therefore, when migration is proceeding normally under 

 safe conditions, very little movement is visible to the ground observer 

 but a large arrival of birds on the ground often indicates something is 

 not in order and the migrants have been forced to stop for one reason 

 or another. 



The question is frequently asked: "How can I identify weather 

 conditions suitable or unsuitable for migration?" It is almost 

 impossible to discuss separately the effects of different weather 

 factors on migration because barometric pressure, temperature, 

 wind, and other meteorological phenomena are very closely related. 



On the North American continent, air masses generally proceed 

 about 600 miles per day from the west to the east. These air masses 

 vary in pressure, temperature, humidity, and wind. The wind within 

 these masses travels in either a clockwise (anticyclonic) or 

 counterclockwise (cyclonic) direction. Cyclonic air masses contain 

 relatively moist warm air with low barometric pressure centers and 

 are designated "lows"; anticyclonic air masses are characterized by 



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