The rock and spruce country of the Pre-Cambrian Shield 

 is avoided by most migrant waterfowl. 



havior." Whether the current is of air or water, the animal moving ran- 

 domly must travel with the flow of the medium in which it wanders. 



Just as a boatman in a great current may avoid shoals by steering 

 around them, so juvenile birds are ecologically selective in their travel 

 within the air mass. If the movements of waterfowl were in complete 

 obedience to the flow of air, there would be a mass passage of western 

 ducks over the inhospitable spruce forests of the Pre-Cambrian Shield, for 

 this vast region is southeast of important duck-breeding grounds. While 

 there is, indeed, some crossing of the Shield, we find the major trend of move- 

 ment holding to the prairies. Sigurd Olson, who has lived many years 

 within the Spruce Forest of northern Minnesota, writes me that he has 

 "observed the flight of ducks in the Pre-Cambrian Shield from the Lake of 

 the Woods to Lake Superior for 35 years and can say that there is no 

 comparison with the flight over the prairies to the west. Along the western 

 edge of the Shield there is a transition area where the flight may be fairly 

 heavy, but as one penetrates more deeply into the Shield country, the 

 numbers become appreciably less." In the same way, autumn juveniles may 

 avoid drought areas or congregate in regions abundantly supplied with 

 water or where there is a profusion of preferred food. 



However the young waterfowl arrives on its wintering grounds — in 

 company with experienced companions, or in the course of its wander- 

 ings—the juvenile's vernal return to the nesting place is to the familiar 

 range of youthful experience. In view of the return of the young to the 

 marsh of their first liberation (as in the experiments of Valikangas, 1933; 



211 



