Figure 25. When a bird flies at random, 

 it must inevitably move in the direction 

 of the wind. Here the speed of the wind 

 is 15 m.p.h. from the northwest. The air 

 speed of the bird is 30 m.p.h. If it flew 

 one hour directly northwest into the wind, 

 one hour southwest, one hour southeast, 

 and one hour northeast, the product of 

 the four hours' travel would be 60 miles 

 southeast of the starting place. 



nance of the Polar Continental Air Mass in September and October must 

 make for a southeasterly movement of these young birds, just as the blind- 

 folded bird, flying in circles, drifts downwind. I believe that all young 

 waterfowl which do not join mass migrations are inevitably destined to 

 travel toward the equator in autumn by the dominant flow of the air mass 

 in which they move. Williamson (1953) explains "that in the case of Green- 

 land-Iceland populations down-wind drift, without dependence on an in- 

 nate sense of orientation, provides the surest method of reaching the west- 

 ern sea-board of Europe in autumn." 



We find evidence of this influence of the wind in other places with 

 other birds. In summer there is a flow of air to the northeast along the 

 Adantic Coast (Trewartha, 1937:88). Young herons of several species, 

 wandering aimlessly from their birth range in the southern United States, 

 arrive in regions far to the north in August and September. Broley (1947), 

 studying the movements of immature Bald Eagles he banded in their natal 

 state of Florida, found a similar trend northward along the Atlantic Coast. 

 Hoar (1953:440) has shown how the Chum Salmon fry move with the 

 current during the night when in darkness they lose their visual orienta- 

 tion,* and he points out that "it is not necessary to assume that fish trans- 

 ported in this way will move at the same rate as floating objects." He goes 

 on to say that "the downstream movement of juvenile and spent fish, em- 

 phasized by Russell (1937), is recognized as a part of the migratory be- 

 havior of a great many fish. Cod, salmon, herring, mackerel and flatfish 

 may be mentioned as a few of the important species showing such be- 



• See p. 69. 



209 



