TRAVELS OF WATERFOWL 



moving to stubble against heavy wind at a ground speed of only 4 m.p.h. 

 In the face of winds that approach their air speed, ducks and geese may re- 

 main grounded in sheltered situations, and several times in periods of strong 

 south winds I have known Mallards to forego their afternoon feeding flight. 

 Often, however, these stubble birds ride the north wind for a quick trip to 

 the fields, then are required to "inch" their way back to the marsh after 

 their meal. It is common knowledge amongst gunners that waterfowl mov- 

 ing against a strong wind fly much closer to the ground than usual. I once 

 met Mallards flying against a 40 m.p.h. wind only ten or fifteen feet above 

 the ground, some birds dropping almost to the field. It is known that ground 

 friction reduces the speed of the wind near the earth's surface, and it is 

 reasonable to assume that the low-flying birds seek the slower currents of 

 low altitude. 



The flow of the air over the earth is not without variation; hence there 

 are frequent changes in the relation of the bird to the air mass in which it 

 travels. Because of hills and valleys, buildings, trees and other obstacles, and 

 also because of differential warming of the land, as between lake and shore 

 or plowed field and cropland, the constant flow of the wind is often inter- 

 rupted by eddies and gusts. "Such gusts of wind are often sudden and vio- 

 lent and it is sometimes true that birds near the earth's surface do, for a 

 fraction of a second, get struck by a gust with a consequent lifting of feath- 

 ers. But the inertia of a bird is so low that it almost instantly assumes the 

 velocity of the gust and is once more in a dead calm" (Smith, 1945:154). 

 This feather-lifting, of course, may occur on the top of the wing and the 

 back when a bird slows its flight close to the point of stalling. Then "the air 

 flow no longer follows the contour of the wing's upper surface, and a con- 

 dition known as 'flow separation' has taken place. When this occurs, a 

 partial vacuum on the upper wing surface results and the coverts are 

 pushed out of place by a reversed air flow rushing over the area" ( Queeny, 

 1947:108). We may have visual evidence of gusts, as where ducks cross the 

 lake ridge to rise abruptly in the face of the updraft over the shore. Wood- 

 cock (1940) tells how gulls at sea search for convection eddies and when 

 found "one sees them change abruptly from a wing-flapping horizontal 

 flight to a steep climb." Human beings experience this effect of turbulence 

 when traveling in an aircraft. The passenger perceives the plane's entrance 

 into a rising column of air as a "bump," but this awareness of the new air 

 mass is fleeting, lasting only for that instant when the shift is made from one 

 air mass to the other. 



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