During spring migration, weather conditions conducive to strong movements of birds are somewhat the opposite from those in the fall. Migrants will move north on the warm sector of an incoming low. When a high pressure area has just passed, the influx of warm moist tropical air is extended and intensified (Bagg et al. 1950). However, during this time, cloudiness and rain associated with the low may curtail migration or squeeze it into a narrow period proceeding along the warm front. If a fast moving cold front approaches from the northwest, the rapid movement of migrants will be sharply curtailed or even grounded until more favorable conditions occur.
The incessant crescendo note of the ovenbird is ordinarily associated with the full verdure of May woods, but 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° between highest and lowest temperatures to 37°, the average variation being about 24°. North American species spending the winter months in tropical latitudes experience no marked changes in temperature 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 time 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 reproductive impulse awakens, and each individual bird is irresistibly impelled to start the journey that ends in its summer home.
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 dependent on proper wind conditions for migration. In the fall, often the best day to observe hawk migration in the eastern United States is on the second day after a cold front has passed providing there are steady northwest to west winds and a sunny day for production of thermals (Pettingill 1962). 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. Haugh and Cade (1966) found most hawks migrated around Lake Ontario when winds were 10 to 25 miles per hour, but, if the wind exceeded 35 miles per hour, most hawk migration stopped.
In conclusion then, we can say that the weather may be the impetus for migration for many species, but it cannot stimulate a bird to migrate unless it is physiologically prepared. Arrivals on the ground are not necessarily indicative of the number of birds passing overhead. During the fall, peak migrations usually follow the passage of a cold front when the temperature is falling, the barometer is rising, winds are from the west or northwest, and the sky is clearing. In the spring, most migrants proceed north in the warm sector of a low when winds are southerly, warm, and moist, but rain, fog, or snow will often curtail the passage of migrants or prevent the initiation of a migration. Evolution of migratory behavior has probably resulted from the survival of birds capable of selecting those wind conditions, which reduce flight time and energy consumption, during their passage.
INFLUENCE OF TOPOGRAPHY
The relation of the world's land masses to each other and the distribution and association of biotypes within these land masses influence the direction birds migrate. Topography may aid, hinder, or prevent the progress of a migrant depending on the bird's particular requirements. Old World migrants must contend with east-west tending mountain ranges and deserts, whereas New World travelers can proceed north and south across a landscape with its major mountain ranges and river systems oriented in the same direction as the birds migrate.
When a distinct feature in the landscape, such as borders between fields and forests, rivers, mountain ridges, desert rims, or peninsulas, appears to influence migratory travel, we call these formations "guiding lines," "diversion-lines," "leading lines," or in German, "Leitlinie." It is an observed fact that some birds in a migratory movement alter their course to travel along a leading line, but whether this feature in the landscape caused the migrants to change their course is only theory (Thomson 1960). Besides topography, many other factors can influence this type of flight behavior including weather, wind speed and direction, time of day, species, age, and experience of the bird (Murray 1964).