MIGRATIONS OF WATERFOWL 



but before nightfall the first Mallards, and sometimes Pintails with them, 

 have made their return. I gather that these birds started out at sundown not 

 far south, and reached Delta before the light failed. Later on in April, great 

 numbers of migrants are seen arriving during the morning. If the rule of 

 evening departure holds elsewhere as it does at Delta, I presume that these 

 birds have come long distances on an overnight journey. 



As in fall, there are "waves" of spring migrants, great numbers moving 

 away at the same time. The timing of these waves closely obeys the weather. 

 South wind and rising temperature mark the onset of the heaviest spring 

 migrations, and a glance at the weather map reveals that this pattern is an 

 exact turnabout of the autumn one precipitating the mass movements of 

 October and November ( Figure 17 ) . The high-pressure area is now located 

 on the eastern side of the continent with the low opposing it in the west, 

 the clockwise flow of air around the high and the counterclockwise stream 

 around the low creating the trough of southerly winds moving up the middle 

 of the continent. This, of course, is the situation so clearly described by 

 Bagg and his cooperators ( 1950 : 13 ) : "northward movement of migrants in 

 late winter and spring will normally begin under conditions of a barometric 

 gradient falling from east to west and of southerly winds typical of the west- 

 ward portion of a high pressure area (clockwise circulation) moving off to 

 the east or southeast. . . . When a high pressure area is supplemented by 

 a low pressure area ( counterclockwise circulation ) originating in the south- 

 west and moving northeastward, the influx of warm, moist tropical air is 

 extended and intensified; concurrendy, the northward movement of mi- 

 grants assumes the proportions of a pronounced onrushing wave in the 

 warm sector of the low pressure area." Schenk ( 1931 ) has described a similar 

 weather condition in Europe where, with the "low" over England and warm 

 air currents moving up from the Mediterranean, the pattern is ideal for 

 spring migration of Woodcock. 



The importance of this meteorological arrangement for mass "pressure 

 pattern" migration was strikingly manifest during the spring of 1954, when 

 the east-west juxtaposition of the "high" and the "low" occurred only once 

 during the whole of April. It was only then that we saw a mass April move- 

 ment of waterfowl and other birds through the Delta region (Figure 17). 

 During the balance of the month the weather was dominated by a strong, 

 stable high-pressure area in northern Canada and, though migration was 

 not wholly stopped, the travel of waterfowl to and away from Delta was 

 thin for the balance of the month. 



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