ENVIRONMENT, PAST AND PRESENT 55 



warblers, thrushes and insectivorous birds generally, 

 or those depending on open water for sustenance, 

 could last through even a week of it. They must 

 depart or perish. But the food situation is not 

 quite so patent in other cases. There is, for instance, 

 the large group of seed-eating birds, which leaves us 

 almost to the last species in spite of the fact that the 

 average snowfall throughout most of the Northwest 

 is very light and there is probably no year in which 

 a virtually unlimited supply of food is not available. 

 Seed-eaters will be further discussed below. 



But there are several sides to the food question. 

 The colder the weather gets, the more food does a 

 bird demand and if it needs more food, it requires 

 more time to procure it. The midwinter day in the 

 Edmonton latitudes is only 9 hours. Further north 

 it is still shorter. There must be a point somewhere 

 at which a junco, for instance, could no longer collect 

 sufficient food to meet its requirements in the hours 

 available even were the supply unlimited. The 

 colder the weather the further south would that 

 point lie. Moreover, when the temperatures be- 

 come extreme (about -30°F. and below) the birds 

 become markedly lethargic. But this is just the 

 time when extra energy is demanded for the col- 

 lection of extra food. At these temperatures it is 

 generally easy to catch any birds in my aviaries by 



