far below zero as long as abundant food is provided. The main 

 consideration then, is depletion of the food supply, caused by either 

 the disappearance or hibernation of insects or the mantle of snow or 

 ice that prevents access to seeds and other food found on or close to the 

 ground or submerged in water. Also, shortened hours of daylight 

 may restrict the ability of birds to obtain sufficient food at a time 

 when low temperatures require increased energy to maintain body 

 heat. It is noteworthy that some of our smaller birds, such as the 

 chickadees, can withstand a cold winter because their food supplies 

 are always available above ground on trees. When there is a good 

 supply of pine and spruce seeds, red-breasted nuthatches and 

 crossbills will remain through the winter in Canadian woods, but 

 when these birds appear abundantly in winter at southern latitudes, 

 it may be concluded there is a shortage of these foods in the North. 



11 



