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

 innate within a species or population that the individuals move north 

 in spring when the average weather is not unendurable. The word 

 "average" must be emphasized since it appears the migrations of 

 birds have evolved in synchrony with average climatic conditions. 

 More northern nesting populations of species such as American 

 robins and savannah sparrows, timed to arrive on their breeding 

 ground when the weather is suitable, pass through areas where their 

 more southern kin are already nesting. 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. Occasionally this results in some 

 hardship, but rarely in the destruction of large numbers of 

 individuals after arrival. Cases are known where early migrating 

 bluebirds have been overwhelmed by late winter storms. 

 Nevertheless, if such unfavorable conditions are not 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. 



The spring flight of migrants, if interrupted by cold north winds, is 

 resumed when weather conditions again become favorable, and it is 

 probable that all instances of arrival in stormy weather can be 

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

 was auspicious. Even though major movements of migrants in spring 

 generally coincide with periods of warm weather and southerly 

 winds, observations on the beginning of nocturnal spring flights 

 from the coast of Louisiana failed to note any inhibiting factor other 

 than hard rain (Gauthreaux 1971). 



Radar studies have indicated that migrant birds possess an 

 amazing understanding of wind patterns (Bellrose 1967). Birds can 

 recognize many characteristics and select for favorable patterns. 

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

 because 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, large numbers of birds are lost. Moderate tail winds 

 and cross or quartering breezes appear to offer the best conditions for the 

 flight of migrants. Richardson (1971) found migrants traveling in 

 different directions at different altitudes, but each group of birds 

 was aided by a following wind. Thus we might expect natural 

 selection to operate in favor of those birds that could recognize and 

 respond to favorable wind patterns because it would reduce energy 

 consumption and flight time on long-distance flights (Hassler et al. 

 1963). 



Soaring birds such as hawks, vultures, and storks are very 



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