MIGRATIONS OF WATERFOWL 



vations estimated at 1,000 to 1,500 feet. Throughout the morning we were 

 aware of a steady flow of wind from the northwest at about 10 m.p.h. ; and 

 yet the weather records of Macdonald, Manitoba, close by, showed that at 

 the elevation the birds traveled, the wind was from the southeast. 



Birds migrate higher with the wind behind than with the wind in face, 

 just as in local travel (Deelder and Tinbergen, 1947). It is agreed that 

 ground friction may reduce the wind near the earth so that in facing the 

 weather, it is an advantage for the traveler to fly low. I'm sure, however, 

 that this is not the whole story. I believe that the apparent ground flow has 

 something to do with the elevation of travel. For example, at an air speed 

 of 45 m.p.h., the bird with a tail wind of 15 m.p.h. has an awareness of 

 ground flow at an elevation of 2,000 feet that is precisely the same as that 

 of a bird at 1,000 feet moving at the same air speed against a 15 m.p.h. wind. 

 This relativity of awareness may establish the height of the "ceiling" of 

 migratory flights, for a bird may rise to elevations (as transcontinental air 

 passengers know ) where there is virtually no perception of ground flow, no 

 awareness of velocity or wind drift. In this respect we note that, with the 

 exception of soaring species, the slow fliers, like blackbirds, Crows, and 

 Flickers, migrate much closer to the ground than swift fliers, like waterfowl 

 and shore birds. 



Weather favorable for diurnal travelers is good for night migrants, and 

 often, after a heavy daytime passage of blackbirds, Flickers, and others at 

 Delta, the night sky is alive with the voices of those species that choose to 

 make their way homeward in darkness. Lincoln (1950a: 29) has pointed out, 

 as is evident to all who have taken night passage by plane, that "the nights 

 are rarely so dark that all terrestrial objects are totally obscured, and such 

 features as coastlines and rivers are just those that are most likely to be 

 seen in the faintest light, particularly by the acute vision of the bird and 

 its aerial points of observation." At Delta most waterfowl migration, spring 

 and fall, begins late in the day. In spring, twilight lingers so long after sun- 

 set on the northern prairies that in late April and early May there is only 

 a short period when there is not a sunglow in the sky. In autumn, however, 

 ducks departing close to sundown strike complete darkness before they are 

 long on their way. 



Lowery (1951) has developed a program for plotting the night migra- 

 tions of birds by watching their movements against the moon with tele- 

 scopes. Moon-watching has been followed for years, but this work brings 

 new techniques for recording and interpreting such observations. This type 



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