THE INFLUENCE OF BAD WEATHER 



August departure of Pintail drakes, the mid-October travel of Canvasback 

 and Redhead from the prairies, and the freeze-up passage of Mallard and 

 Lesser Scaup, every hunter on the southern ranges knows that some years 

 waterfowl do not come in their regular volume or at their usual time. In 

 spring, too, there are years when the homeward passage is many days later 

 than normal. 



Major changes in the weather will alter the time, the place, and the 

 volume of migration. During periods of drought, main flights of birds may 

 shift to country more abundantly watered, and an unseasonably wet year 

 may bring concentrations of ducks unusual to a certain region. In 1946 and 

 1947 there were heavy flights of White-fronted Geese through central Mani- 

 toba where the species had been uncommon. Hunters farther west in Sas- 

 katchewan, where the region was drier than usual, reported below-normal 

 flights of white-fronts. In 1940 winds blew steadily from the southeast at 

 Delta throughout late September and early October. Arrivals from the north 

 during this period were so few that the success of the local hunters was 

 greatly reduced and the small bag was the outstanding feature of the sea- 

 son. In 1943 south wind was unrelenting during the middle of October 

 when the Canvasback and Redhead generally leave Delta, and these species 

 were common until the twenty-second, when the wind abated and the birds 

 moved southward. 



Flights may be deflected by bad weather, as in 1949, when there was 

 almost no spring migration through Delta, but at the Lower Souris Refuge, 

 in North Dakota, there was a heavy passage. McCreary (1934) tells how 

 a change in the normal pattern of winds caused a major shift in the spring 

 movement of migrants, mostly small birds, in Wyoming. In England, Clarke 

 (1912) observed that migration from Scandinavia was blocked by north- 

 west wind, but winds from the southeast were followed by heavy migration. 

 Over land the direction of migration may follow leading lines, such as rivers 

 or shorelines or mountain ranges, hence continue its normal course despite 

 adverse winds. Williamson (1952), however, considers that birds at sea, 

 especially at night, are far less able to adjust for their wind drift, and then 

 overseas movements may drift far from the regular course. 



After the 1954 avalanche of spring migrants, April 8 to April 10 (see 

 above, p. 119), the weather in Manitoba was dominated by the Polar Con- 

 tinental air mass, with a strong high-pressure area in northern Canada and 

 a stationary Arctic front that held its mean position through the northern 

 part of Lake Manitoba from April 10 to April 17. Then, for the balance of 



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