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.