UTILITY OF BIRDS IX NATURE. 



in a few years would be carpeted with crawling caterpillars, 

 and the moths in flight would cover the earth like a blanket 

 of fog. But under natural conditions the caterpillars that 

 hatch from the eggs of the moth are destroyed by birds, 

 mammals, insects, or other animals, by disease or the action 

 of the elements, so that in the end only one pair of moths 

 succeeds another. If every Robin should jn-oduce five young 

 each year, and each Robin should live fifteen years, in time 

 every square foot of land on this continent would bo packed 

 with Robins ; Ijut the surplus Robins are killed and eaten 

 by various other birds or by mammals, each striving to 

 maintain itself; so that, eventually, the number of Robins 

 remains about the same. 



Thus we see that, while birds, insects, other animals, and 

 plants are constantly striving to increase their nimibers, the 

 creatures that feed ujjon them operate continually to check 

 this undue nuiltiplication. The Hawk preys upon the smaller 

 birds and mammals. The smaller birds and mammals feed 

 on insects, grass, seeds, leaves, and other animal and vege- 

 table food, each virtually endeavoring to gain strength and 

 increase the numbers of its race at the expense of other 

 living^ oro-anisms. 



There is a competition among various dissimilar organisms, 

 also, in seeking certain kinds of food. Grazing mammals, 

 such as cattle, sheep, and deer, eat grass. Grass is eaten 

 also by birds, mice, and insects. If any one kind of these 

 creatures should be left without check, and become too 

 numerous, it might consume the food supply of all. 



In the great struggle for existence, each perpetuating 

 form of life that we call a species is really an expansive 

 force, that can be restrained and kept in its proper place 

 only by the similar expansive forces (other species) by 

 which it is surrounded. It is as if the whole field of ani- 

 mal and vegetable life consisted of a series of springs, each 

 exerting a pressure in all directions, and each held in place 

 only by the similar expansion of the springs surrounding it. 

 This action and reaction of natural forces constitute what is 

 known as the balance of nature. Any serious disturbance 

 of this balance is always fraught with serious consequences. 



