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).