FLIGHT TRAILS SOUTH 



sage went to the same point of the compass as I had marked it for the fall 

 flight in 1938 (Figure 10). While the wind is invariably from the northwest 

 in these main passages, it seldom blows in exactly the same direction as the 

 flight so that the course of the birds is often several degrees east or west of 

 the flow of wind. 



With respect to this firmness of direction in the path of autumn migra- 

 tion, Thomson (1953) reminds us of the "modern view that there is a stand- 

 ard direction * for the migration of each species, or community of species 

 . . . This direction leads either straight from the breeding area to the win- 

 tering area, or to some intermediate area where the direction is assumed to 

 undergo a change. The capacity to follow such a direction is supposed to be 

 innate in the bird, as part of its migratory instinct, and this hypothesis 

 conforms with the experimental findings of Kramer (e.g., 1952) and Mat- 

 thews (1952) that birds are able to orient their flight in relation to the posi- 

 tion of the sun, making due allowance for the time of day." Note that Thom- 

 son refers to the capacity to follow a direction as being innate without im- 

 plying that a bird is innately bound to a certain direction. 



The travelers in these mass passages are most often seen in "waves" of 

 flocks, with several or many bands within sight at one time. The movement 

 is always higher than local travel, usually ranging from 500 to about 5,000 

 feet, with most migrant waterfowl crossing the Delta region between 1,000 

 and 3,000 feet f (Figure 11). Flock size varies from a dozen or so to strings 

 of a hundred or more, most bands containing from twenty-five to fifty birds. 

 During the course of a passage one may see several or many species, but 

 flocks are seldom mixed, each kind traveling in a group of its own. Individ- 

 uals of a migrating flock string out in a line forming a blunt arc or, much 

 less frequently, a sharp-pointed V, each bird thus avoiding the slipstream 

 of rough air produced by the movement of its companions. 



The mass migration over the Delta Marsh moves in a broad sweep. On 

 a day of a grand passage, hunters scattered over the full width of the marsh 

 report heavy movements; and on such days at Delta, W. A. Murphy has 

 told me of spectacular flights over his East Meadow Ranch on the eastern 

 shore of Lake Manitoba. The width of this passage is probably thirty miles, 



Thomson's phrase here comes from Geyr von Schweppenburg, who at first ( 1933) used the 

 term Normalrichtung and later ( 1948 ) Primarrichtung, the latter translated by Deelder ( 1949) 

 as "standard direction" (van Dobben, 1953). 



t All statements of elevation given here are based on estimates of two sorts : ( 1 ) by com- 

 parison with relative-size standards from exact measurements of duck and goose silhouettes 

 (Figure 11); (2) by triangulation. Both field techniques are subject to error, but I feel they 

 are sufficiently in line to evaluate approximate elevations. 



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