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.
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. Soaring birds such as hawks, vultures, and storks are very dependent on proper wind conditions for migration. 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.