MIGRATIONS OF WATERFOWL 

 November 9 he found no ducks remaining around Fergus Falls, and the 

 area was frozen over except for a few open holes. All the birds of the Cana- 

 dian prairies and the northern states apparently cleared out in this big push. 

 The mass migrations we witness at Delta are related to four phases of 

 the weather: air pressure, wind, temperature, and humidity. Without im- 

 plying anything as to cause and effect, I have found that a rising barometer, 

 a falling temperature, a drop in relative humidity, and a wind from the 

 northwest establish the weather pattern characteristic of the mass migrations. 

 Indeed, more than once I have predicted the grand passage on the basis of 

 this weather combination. 



In a discussion of these weather characteristics, Robert L. Lillestrand, of 

 the General Mills Research Laboratories, pointed out how the relation of 

 winds to the high-pressure (anticyclonic) and low-pressure (cyclonic) air 

 masses might favor southward migration when a "low" passed Delta and a 

 "high" arrived. The clockwise movement of air around the high together 

 with the counterclockwise rush around the low creates a southerly flow of 

 wind. Lillestrand drew a theoretical weather map to illustrate this situation, 

 with the high-pressure area located on the western half of the continent and 

 the low in the east, the trough of winds flowing down the middle between 

 the two air masses. I then examined the weather maps coinciding with the 

 major avalanches of fall passage and found that for Blue-winged Teal in 

 September, Canvasbacks and Redheads in October, and Lesser Scaup and 

 Mallard in November, every major flight took place when there was such a 

 west-east opposition of the high- and low-pressure areas. I have selected the 

 grand passage of November 7, 1947, to illustrate this weather pattern so 

 favorable to the mass movement of autumn migrants (Figure 14). 



With respect to this tie between the weather and migration, Landsberg 

 (1948:709) believes that migratory birds "have developed a rather remark- 

 able system of what is called in modern aviation pressure-pattern flying/ 

 This is the system that takes advantage of the maximum possible amount of 

 tail wind in long-distance flights." Bagg (1948:147) likewise suggests that 

 there is a strong barometric pressure-pattern in North American weather 

 that stimulates migration. While the southward flow of wind is perhaps the 

 outstanding feature of this favorable weather situation, I cannot believe that 

 it is the only influence starting off these mass movements. I have never 

 known heavy migrations to start out from Delta during the cloudy, unsettled 

 weather immediately preceding the arrival of the high-pressure air mass, 

 even though the wind blew from the northwest, the temperature dropped 



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