THE EVOLUTION OF MIGRATIONS 91 



Alberta, and a certain number of birds always stay 

 where open water and food remain available. In 

 years in which the fall is late and open, a far larger 

 number stay behind. Alberta mallards are mainly 

 grain eaters in the fall and, regardless of the type of 

 water they may be frequenting (i.e. it may, or 

 may not, provide food) they have our expansive 

 wheat fields to afford them unlimited sustenance. 

 They appear to be unaffected by severe cold but 

 should a heavy snowfall obliterate the fallen grain 

 in the stubble, they speedily starve to death unless 

 artificially fed. In this particular instance, failure 

 of their food supply is the lethal factor. Central 

 Alberta alone may be snow covered, or the entire 

 Province, or even the States south of the Canadian 

 boundary. Every normal winter sees the exter- 

 mination of a certain number of mallards but every 

 winter, on the other hand, also sees the survival of 

 others, birds, for instance, that frequent certain 

 hot-springs that never freeze over and from which 

 they derive an adequate food supply. These 

 birds do not go south and as long as they survive 

 and reproduce they will add to the number of mal- 

 lards that may be termed resident. The number 

 can never increase beyond certain limits because 

 the favorable waters are restricted and those win- 

 tering elsewhere are more than likely to be wiped 



