xii CONTENTS 



B. Their Economic Value 



VII. Friends among the Birds as Destroyers of 



Insect Pests 63 



Bureau of Biological Survey — Methods of determin- 

 ing food of birds — -Harm done by insects — Nature's 

 check on insects — Service performed by birds — 

 Amount of food eaten by birds — Value of nestling birds 



— Control of insect outbreaks — Division of labor — 

 Birds of a Maryland farm — Birds and human health. 



VIII. Friends among the Birds as Destroyers of 



Weed Seeds 84 



Harm done by weeds — Characteristics that make 

 weeds troublesome — Birds as destroyers of weed seeds 



— Amounts of weed seeds eaten — Effect on weed- 

 patches. 



IX. Birds as Destroyers of Rodent Pests . . 91 

 Harm done by rodent pests — ■ Value of hawks and 

 owls — Study of food of hawks and owls — Classifica- 

 tion of hawks and owls — Money value of hawks and 

 owls. 



X. Foes among the Birds 99 



Fruit-eaters — Injury to grain — Destruction of 

 poultry — Damage to trees and wood products — De- 

 stroying beneficial insects — Injury to valuable birds 



— Preventing depredations of birds. 



XI. Striking the Balance 110 



Harmful birds — Neutral birds — Beneficial birds 



— Table of food of birds. 



PART II 



Enemies of the Birds 



XII. Changes in the Numbers of Birds . . .116 

 Extinct birds — Species in danger of extermination 

 — Decrease of game-birds and shore-birds — Have song- 

 birds decreased? 



XIII. The Natural Enemies of Birds . . . .128 



Four-footed enemies — Feathered enemies — The 

 elements. 



