16 University of California Publications in Zoology [Vol.11 



forms of the total food taken by the bird population is less 

 when the insects are in normal numbers, yet it is possible that 

 this small amount of destruction is more important as a means 

 of check than a larger proportion in the food taken when the 

 insects are in abnormal numbers. The average number of grass- 

 hoppers, when in normal numbers, per square yard probably 

 does not exceed two or three, and as a rule is probably less. The 

 bird population, though taking but a tenth as many grasshoppers 

 at such a time, would be taking a far greater percentage of the 

 total number of these insects than when taking the numbers 

 found to be consumed during the outbreak. A smaller number 

 of grasshoppers destroyed at the time of minimum numbers has 

 a more important bearing on the prevention of an increase than 

 a larger number destroyed at the time of maximum numbers. 

 We can safely infer, therefore, that the regulative influence of 

 birds is just as important throughout the year as during an 

 insect outbreak, or even more important. 



The failure of birds to check an insect outbreak after insects 

 have appeared in abnormal numbers, is evident to all. Their 

 success in preventing insects from becoming abnormally abund- 

 ant is not at once so apparent. No convincing demonstration 

 of their good work in this connection is obtainable for lack of 

 control experiments. Their constant regulative action must be 

 inferred from data regarding the food of birds and their rela- 

 tion to insects when they appear in abnormal numbers. Judg- 

 ing from all the evidence obtainable, we are justified in believing 

 that birds exert an important regulative influence on insect life 

 throughout the year. And this regulative influence appears to 

 keep pace with the fluctuations of insect life. The importance 

 of birds because of their regulative influence on insects, there- 

 fore, although more apparent at the time of an insect outbreak, 

 is no less important when insects are to be found in normal 

 numbers. 



Emphasis can well be placed on the fact that a diminution 

 of the numbers of an injurious in.sect must cause a corresponding 

 diminution of the damage done. If twenty grasshoppers are 

 causing damage on a square yard of alfalfa the loss of even two 



