18 THE RELATION OF 9PABBOWS TO AOBIOULTUBE. 
entirely understood. The difficulties involved are well illustrated by 
certain observations made by l>r. 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 (Schizo- 
ii) urn lantgera). ] Tins was. of course, a beneficial effect rendered by 
the birds, but at the same time they were lulling the larvae of the 
ladybirds, lacewings, and syrphus (lies, which were also destroying 
the plant-lice. It would be necessaryto 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 winch birds become 
pests, and by inspection of the food elements of sparrows through 
the different methods of investigation heretofore described, more 4 
especially through the combination of field work with stomach exami- 
nation, and further by 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. 
