18 THE RELATIOK OF SPARROWS TO AGRICULTURE. 



entirely understood. The difficulties involved are well illustrated by 

 certain observations made by Dr. J. A. Allen. He found the tree, 

 chipping, field, and white-throated sparrows, and the junco preying 

 upon an insect pest of the apple, the apple-tree plant-louse [Scliizo- 

 neura lanigerd)} This was, of course, a beneficial effect rendered by 

 the birds, but at the same time they were killing the larvse of the 

 ladybirds, lacewings, and syrphus flies, which were also destroying 

 the plant-lice. It would be necessary to ascertain to what extent the 

 evil effect of killing the enemies of the plant-louse counterbalanced 

 the good effect of killing the plant-louse itself before the final effect 

 of the sparrows upon apple culture could be determined. 



RECAPITULATION. 



By keeping in mind the exceptional ways in which birds become 

 pests, and \)j inspection of the food elements of sparrows through 

 the different methods of investigation heretofore described, more 

 especiallj^ through the combination of field work with stomach exami- 

 nation, and further hy the classification of these elements of the food 

 into their neutral, beneficial, and injurious categories the effect of 

 sparrows on cultivated crops can be approximately ascertained. 



1 Vide B. D. Walsh in The Practical Entomologist, Vol. II, No. 4, p. 46, 1867. 



