CHAPTER VI. 



BIRDS AS REGULATORS OF OUTBREAKS OF INJURIOUS 

 ANIMALS. 



IT is well known that, as a rule, the most damage to culti- 

 vated crops is done by the species of insects and other 

 noxious animals which fluctuate greatly in numbers. In this 

 chapter we have to determine whether in the presence of an 

 extraordinary abundance of a given edible animal birds vary 

 their food ratios by taking unusual numbers of the species in 

 question. If they do, it is evident that they assist in reducing 

 the pest to its normal limit ; if they do not, they neglect an 

 opportunity for usefulness. 



Four examples may serve to illustrate the tendencies of 

 birds under such conditions. The first relates to the canker- 

 worm, the second to the Rocky Mountain locust, the third to 

 the army-worm, and the fourth to the European vole, or field- 

 mouse. 



A few years ago a large apple-orchard in central Illinois 

 was severely attacked by canker-worms. As a result of their 

 depredations a considerable part of the orchard had the ap- 

 pearance at a little distance of having been ruined by fire. 

 To determine whether the birds of the region were exerting 

 themselves to check this outbreak, Professor S. A. Forbes 

 visited the orchard for two successive seasons, shooting each 

 time a number of birds of the various species present. The 

 stomach contents of these were afterward carefully examined : 

 from the published record l of the results we have made the 

 following summary. 



1 Bulletin of the Illinois State Laboratory of Natural History, vol. i., 

 No. 6. 

 68 



