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 BlRDS 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. 



