panding and contracting springs, each of which in expanding is 

 checked, pushed back or compressed by others. The moment 

 one weakens and becomes slightly contracted others expand to 

 fill the vacancy. When one expands unduly, others exert in- 

 creased force to contract it again; for example, when locusts 

 become unduly numerous and devastate the land, practically all 

 wild beasts and birds neglect other foods and consume locusts 

 until the latter again become reduced in numbers. All the 

 forces of Nature are thus balanced one against another. Plants 

 and trees produce foliage and seed enough to feed all living 

 animals, some of which take their food direct from plant life, 

 while others get it wholly or in part at secondhand by devour- 

 ing insects or other animals which feed on plants. 



We may say, then, that in the economy of Nature all species 

 are useful, since all have their part in preserving that general 

 balance and stability which works for the good of all life. 



The Function of Birds in Nature. 



The chief value of birds in the general plan lies in the great 

 part that they have in maintaining this biologic balance, a 

 part that cannot be filled by other creatures. Like many other 

 organisms they are ordinarily rather impartial of choice regard- 

 ing food, and they forage wherever and on whatever is best for 

 the common welfare. Nevertheless, birds exercise some choice 

 and fill a special place. Their position in one respect is unique. 

 Their structure fits them to perform a certain function, that 

 of a swiftly moving body of police, adapted to sweep rapidly 

 over the surface of the earth and assemble quickly in hosts 

 wherever most needed to combat abnormal outbreaks or ir- 

 ruptions of animal life. 



A swarm of grasshoppers appears, and birds from far and 

 near concentrate upon them. A plague of field mice occurs, 

 and birds descend upon them from the four quarters of the 

 land. This facility of movement renders birds serviceable, also, 

 in clearing the earth of offensive decaying animal matter, for 

 many are scavengers. Quantities of fish are cast upon the shore, 

 and thousands of sea birds come sweeping in from wide waters 

 and far shores to devour them. Vultures gather from afar to 



