Influence of the Weather on Migration 



The state of the weather at any point has Httle if anything to do with 

 the time of arrival of migratory birds. This is contrary to the beHef 

 of observers who have thought that they could foretell the appearance 

 of various species by a study of the weather conditions. Though the 

 insistent crescendo note of the ovenbird is ordinarily associated with 

 the full verdure of May woods, this bird has been known to reach its 

 breeding grounds in a snowstorm and the records of its arrival in 

 southern Minnesota show a temperature variation from near freezing 

 to full summer warmth. Temperatures at arrival of several other 

 common birds vary from 14 degrees between highest and lowest tem- 

 peratures to 37 degrees, the average variation being about 24 degrees. 



It should be remembered that North American species spending the 

 winter months in tropical latitudes experience no marked changes in 

 climatic conditions from November to March or April, yet frequently 

 they will start the northward movement in January or February. 

 This is in obedience to physiological promptings and has no relation 

 to the prevailing weather conditions. For migratory birds the winter 

 season is a period of rest, a tin c when they have no cares other than 

 those associated with the daily search for food or escape from their 

 natural enemies. Their migrations, however, are a vital part of their 

 life cycles, which have become so well adjusted that the seasons of 

 travel correspond in general with the major seasonal changes on their 

 breeding grounds. With the approach of spring, therefore, the repro- 

 ductive impulse awakens, and each individual bird is irresistibly im- 

 pelled to start the journey that ends in its summer home. 



In other words, the evidence indicates that the urge to migrate is 

 so ingrained that each species moves north in spring when the average 

 weather that will be encountered is not unendurable. The word 

 "average" must be emphasized since it appears obvious that the migra- 

 tions of birds have so evolved that in general they synchronize with 

 average climatic conditions. The hardy species travel early, fearless 

 of the blasts of retreating winter, while the more delicate kinds come 

 later when there is less danger of encountering prolonged periods of 

 inclement weather. Some of the hardy birds pause in favorable areas 

 and allow the spring season to advance. Then, by rapid travel they 

 again overtake it, or, as sometimes happens, they actually outstrip it. 



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