244 ENTOMOLOGY 



extraordinary number and variety, there being at least thirty-five 

 species, most of which were studied by Forbes, from whose exhaustive 

 tables the following food-percentages are taken: 



BIRDS INSECTS, CANKER-WORMS, 



EXAMINED PER CENT. PER CENT. 



Robin 9 93 21 



Catbird 14 98 15 



Brown Thrush 4 94 12 



Bluebird 5 98 12 



Black-capped Chickadee 2 100 61 



House Wren 5 91 46 



Tennessee Warbler i 100 80 



Summer Yellow Bird 5 94 67 



Black- throated Green Warbler. . . i 100 70 



Maryland Yellow-throat 2 100 37 



Baltimore Oriole 3 100 40 



To quote Forbes: " Three facts stand out very clearly as results of 

 these investigations: i. Birds of the most varied character and habits, 

 migrant and resident, of all sizes, from the tiny wren to the blue-jay, 

 birds of the forest, garden and meadow, those of arboreal and those of 

 terrestrial habits, were certainly either attracted or detained here by the 

 bountiful supply of insect food, and were feeding freely upon the species 

 most abundant. That thirty-five per cent, of the food of all the birds 

 congregated in this orchard should have consisted of a single species of 

 insect, is a fact so extraordinary that its meaning can not be mistaken. 

 Whatever power the birds of this vicinity possessed as checks upon 

 destructive irruptions of insect life was being largely exerted here to 

 restore the broken balance of organic nature. And while looking for 

 their influence over one insect outbreak we stumbled upon at least two 

 others, less marked, perhaps incipient, but evident enough to express 

 themselves clearly in the changed food ratios of the birds. 



" 2. The comparisons made show plainly that the reflex effect of this 

 concentration on two or three unusually numerous insects was so widely 

 distributed over the ordinary elements of their food that no especial 

 chance was given for the rise of new fluctuations among the species 

 commonly eaten. That is to say, the abnormal pressure put upon the 

 cankerworm and vine-chafer was compensated by a general diminution 

 of the ratios of all the other elements, and not by a neglect of one or two 

 alone. If the latter had been the case, the criticism might easily have 

 been made that the birds, in helping to reduce one oscillation, were 

 setting others on foot. 



"3. The fact that, with the exception of the indigo bird, the species 



