TRADITIONS OF WATERFOWL 



are separated more by space than by sharp geographical barriers, and their 

 many subspecies blend together from one region to another without abrupt 

 divisions between two races. Where space is the major isolating factor, we 

 have what Mayr (1942:244) calls "subspeciation by distance." "The forma- 

 tion of local races in the absence of distributional barriers will occur," Mayr 

 tells us, "whenever the combined effects of recurrent local mutations and 

 of selection pressure outweigh the effect of the interchange of individuals 

 between local populations." 



Free as the wind to fly wherever it will, a Song Sparrow wintering in 

 Georgia could just as easily fly northeast to the Atlantic Coast as northwest 

 to central Canada ; but by force of experience it travels to its former breed- 

 ing place in Manitoba, where it is geographically isolated from some win- 

 ter companions that migrated to Maine. Its tie to Manitoba can hardly be 

 considered a genetic bond. The return is to the place of experience; and 

 this, as Nice ( 1937 ) has shown for the Song Sparrow, is the region of youth, 

 often near the birthplace. Manitoba is the birthright of its Song Sparrows; 

 their inheritance of Manitoba experiences is traditional. The homing be- 

 havior which brings the migrant back to traditional breeding grounds estab- 

 lishes the geographic isolation upon which racial subdivision depends. To 

 put it another way : if Song Sparrows did not return to their home region 

 year after year, if they spread their breeding ranges widely without regard 

 to birthplace, there would not be the same degree of geographical isolation 

 that now exists, and we could not expect the same racial variation. Cushing 

 (1941a: 107) observes that "if we are to consider the homing reaction as a 

 potential isolating mechanism, it may be classifiable as on par with geo- 

 graphic isolation (see Dobzhansky, Genetics and Origin of Species, 1937: 

 230 ) , although itself physiological in nature. Such non-genetic mechanisms, 

 while presumably of a relatively temporary nature, theoretically precede 

 the rise of genetic isolation within populations." So it may be that in some 

 species the primary factor in isolation is the tie to the home breeding range. 



In species with broad ranges we might expect those kinds possessing 

 the strongest traditional ties to have the greatest racial variations. This 

 holds with the geese. With their firm regional and family traditions, the 

 geese follow a pattern of behavior which must encourage the development 

 of variation. Mayr (1942:242) says that "no other arctic or subarctic bird 

 breaks up into so many pronounced races as the geese. The Canada Geese 

 of the genus Branta have some 6 to 9 geographic races in North America, 

 some of which are so different that some authors propose to put them in 3 



238 



