MIGRATIONS OF WATERFOWL 



Wynne-Edwards (1935) suggests, the waves breaking the surface might 

 cause an air swell to the travelers' profit. Such advantages, however, would 

 not be so keenly enjoyed at elevations of 100 feet or so. I believe that the 

 consistency of low-level flying is also related to the bird's requirement to 

 perceive its relative velocity. Thomson ( 1942 ) points out that in over-water 

 passages, flight "without landmarks . . . seems to leave birds without any 

 means of estimating their lateral drift." Odum (1948), however, believes 

 that birds perceive their ground speed over water, explaining that ripples, 

 swells, and whitecaps, although in motion themselves, give a texture to the 

 water surface that is reference for accurate perception of velocity. "That 

 this is true is known from the standard procedure of aviation over the ocean 

 whereby the wing velocity of the air mass in which the plane is imbedded is 

 told from 'double drift' measurements in reference to the sea surface." An 

 awareness of velocity and drift from the water depends upon the speed and 

 height of flight as well as the texture of the surface water. An aircraft can 

 rise so high that perception of velocity and drift is impossible, in which 

 case the navigator must drop a smoke flare by which to measure his drift. 

 And in a complete calm, the air passenger does not perceive relative velo- 

 city until the plane is almost touching the water surface. Birds, without in- 

 struments or flares, must keep low enough to the water surface for visual 

 perception of velocity. The Pintail flying at 33 m.p.h. at 200 feet would be 

 keenly aware of its relative movement, but at 2,000 feet over the ocean, its 

 perception of velocity and drift would be reduced to nil. If the sun is an aid 

 to navigation, as Kramer and Matthews suggest, then sun-awareness and 

 perception of velocity and drift will be very important cues for the ocean 

 migrant; but an awareness of the sun without perception of velocity and 

 drift will make it much more difficult to pursue an established direction. 



That overseas travelers do hold to a line of flight is evidenced by the 

 observations of Paynter (1953), who found a neat consistency of direction 

 in migrants passing over the Campeche Bank of the Gulf of Mexico. And 

 Siebenaler (1954), also studying migration over the Gulf of Mexico, made 

 an extremely important observation of birds adjusting their line of flight to 

 the wind. "At 10:59 a.m. on October 3, 1952," he writes, "the 'Oregon' was 

 160 miles north of Yucatan, and it was traveling a true course of 180 degrees 

 at a speed of 10 knots. At this time heavy flights of warblers and other land 

 birds were noticed overhead. The wind was from the northeast at speeds of 

 15 to 18 miles per hour. The birds were not traveling at a uniform altitude, 

 but those that came near enough to be seen well with the naked eye or 



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