26 University of California Publications in Zoology. |Vou.9 
lion. Then, as above shown, at least half a million birds must 
succumb to various causes before the next breeding season. For 
if the population remains constant the death rate must equal the 
birth rate. 
The factor that undoubtedly determines the maximum number 
of birds that can exist seems to me to be food supply. The young 
are produced at the beginning of the season of plenty, which, as 
pointed out above, consists of the months of July, August, and 
September. After this, the cold season rapidly comes on and the 
food supply suddenly diminishes to a minimum, probably reached 
at the time of the first snows, say In November. As a result the 
vast bulk of the bird population, all except the few permanently 
resident species, emigrate to lower zones for the winter. The 
numbers of birds, all categories, which can exist throughout the 
year anywhere must be determined by the amount and availa- 
bility of food supply at the season of extremest shortage. With 
permanently resident species, then, such as woodpeckers, nut- 
crackers, nuthatches, ete., the number we find in June is prac- 
tically an index of the maximum food-supplying power of the 
region during the season from November to Mareh. In the ease 
of breeders which have come to the region only for the summer, 
their numbers must have been determined by the sustaining 
power of their winter habitat, wherever that may be. 
The intricate system of summer and winter habitats of migra- 
tory birds I believe to have resulted from a gradual process of 
elimination of those individuals finding themselves in a region 
of relative food seareity, and a preservation and perpetuation of 
those individuals finding themselves in regions of best food con- 
ditions. Competitive struggle between species has led to the 
adoption of remote and otherwise unexplainable habitats, tem- 
porary or constant. It has also led to the development of various 
and perfected means of food-getting. The geometric ratio of 
reproduction makes the population of a species an elastic quan- 
tity, expanding into any favorable food area presenting itself. 
And the masses of different species press against one another, like 
soap-bubbles, crowding and jostling, as one species acquires, 
through modification of food-getting powers und perfected adapt- 
ability to other conditions, some advantage over another. 
