A Continentwide View of Bird Migration on Four Nights in October

Abstract

A series of two-color and multi-symbol charts based on counts of birds passing before the moon show computed amounts and major directional trends of migration over the United States and southern Canada on 4 successive nights in Oct. The charts permit direct comparison of features of the migration with factors potentially affecting them-the speed and direction of winds aloft, the character of surface weather systems, the disposition of temperature inversions and isothermal layers, and the physiography of the North American continent. Migration traffic in the Far West on these 4 nights is found to be generally inferior in quantity to the flights occurring east of the Rockies under favorable meteorological circumstances. Differences in the quantity and average direction of migration from one far-western observation station to another are too inconsistent-and the stations themselves are too sparse-to support any other generalization except possibly that extremely mountainous terrain produces pronounced local variations. In the Midwest and East, heavy night migrations often extend over vast areas, including several states, and display homogeneity with respect to quantitative rating. Similarly, areas with no detected migration are often far-reaching. Sharp contrasts in migration rating between stations close to one another do occur-and sometimes without clearly evident cause. No effect of physiography asserts itself except for indications that a sizable proportion of migrants may detour the Great Lakes and the Gulf of Mexico. In particular, the maps fail to support the idea that migration tends to follow river systems, though the possibility is not precluded that some kinds of migrants may do so. The charts show a variable reaction of migration traffic to the winds aloft selected for comparison. Heavy migration is most frequently associated with following winds, and wind usually seems to affect the direction of migration, even causing reverse movements on occasion; but many exceptions appear. The contention that following winds must be gentle to permit large migrations is contradicted by the present results. Low or falling temperature had no discernible separate influence on the migrations of the four nights except possibly with regard to a movement behind the freeze line in which waterfowl seem to have been prominently involved. Below-average migration traffic was far more frequent than traffic that exceeded the average. The implication is that the greater part of migration through a given area takes place on relatively few nights. The charts furnish no clear indication of the night-to-night advance of migration waves.