Table 5.— The wind direction and the wind velocity in miles per hour for certain hours of the day, November 1 

 or 2, 1955, at several weather stations from southern Manitoba to southern Louisiana. 



*Hour of passage of spectacular waterfowl migration of late October and early November, 1955. 



sure generally attend the departure of migrants." 

 Raynor (1956:171) tentatively concluded that stable air 

 and favorable winds aloft were of great importance for 

 nocturnal bird migration during the spring. 



Hochbaum (1955:104) stated that every major flight 

 of blue-winged teals in September, canvasbacks and 

 redheads in October, and lesser scaups and mallards in 

 November occurred with a west-east opposition of high 

 and low pressure areas, with a trough of wind flowing 

 down the middle between the two air masses. He used 

 the weather map of November 7, 1947, to illustrate the 

 favorable flow of air for the waterfowl migration which 

 occurred at that time. 



Weather maps for October 31 and November 1, 2, 

 and 3, 1955, figs. 3-6, show wind directions and veloci- 

 ties near ground level by means of arrows, and above 

 ground level up to about 2,000 feet above ground level 

 by means of isobars. The wind flows along the isobars; 

 the closer the isobars, the higher the wind speed. The 

 wind circulates counter-clockwise around a low pres- 

 sure area, the reverse around a high pressure area. 



Because, during the mass migration of 1955, water- 

 fowl were migrating between a few hundred and 2,000 

 feet above the earth, they were under the influence of 

 winds shown by both arrows and isobars. The winds 

 were only partially favorable for southward migration. 

 They were not beneficial on October 31 and November 1 

 in Manitoba, the Dakotas, Minnesota, and Wisconsin, 

 when they were largely from the west at almost right 

 angles to the line of flight. Table 5 shows the wind 

 directions and velocities on certain hours during the 

 period the mass migration passed several weather sta- 

 tions in the Mississippi Flyway. Seldom did the migrat- 

 ing waterfowl have a tail wind. 



Wind directions were at least as favorable on 

 October 29 as during the period October 31-November 2; 



yet comparatively little migration occurred on October 

 29. 



Over a large area of flight on November 1-3, 1955, 

 waterfowl were migrating through turbulent air, through 

 snow showers in the northern sector, and under a layer 

 of low, dense, and extensive clouds. Such conditions 

 certainly made for hazardous flight and difficult navi- 

 gation. 



Temperature. —An interpretation of the effect of 

 cold fronts on the mass waterfowl migration of 1955 is 

 difficult because of the complicated frontal structure 

 over the period of the migration. A cold front of Mari- 

 time and Continental Polar air, shown over the north- 

 western United States on the weather map for October 

 31, fig. 3, passed down the flyway to the Midwest as a 

 secondary cold front of Continental Arctic air was mov- 

 ing behind it in southwestern Canada and northwestern 

 United States, fig. 4. The leading edge of the secondary 

 front was so diffuse as to make its position quite un- 

 certain. In fact, the secondary front did not appear on 

 the weather map for November 2, fig. 5. Eventually it 

 combined with the Polar cold front on November 2 north 

 of Memphis, fig. 5. The passage of a secondary cold 

 front at a given point can best be ascertained by tem- 

 perature decline. Usually winds shift with the passage 

 of a front, but such was not the case with the secondary 

 cold front shown on the weather map for November 1, 

 fig. 4. 



The average daily temperatures at Winnipeg, Mani- 

 toba, fell for 4 successive days before the mass mi- 

 gration started from Alberta and Saskatchewan on 

 October 31 and 5 successive days before the migration 

 started from Manitoba on November 1. Temperatures for 

 this period are shown in fig. 7. 



On November 1, the average temperature was 26.0 

 degrees and the minimum temperature 23.2 degrees. 



22 



