BIRDS IN RELATION TO PARASITIC INSECTS. 85 



sequently there is a great and sudden decrease of the numbers 

 of the parasite. 



In other words, while the law that no species can multiply 

 beyond the limits of its food supply is rendered inoperative 

 in the case of the host caterpillar, it continues to act in the 

 case of the parasite, because man does not artificially increase 

 the food supply of the latter. Man's interposition evidently 

 has the effect of extending and intensifying the oscillations 

 which would occur under natural conditions. 



We must next determine the probable effect produced when 

 a bird eats some of these parasites. It need hardly be said 

 that the species of insects which live exposed are very much 

 more likely to be eaten by birds when they are unusually 

 abundant than when not numerous. When a hymenopterous 

 parasite is found in a bird's stomach, the chances are greatly 

 in favor of the assumption that the species to which it belongs 

 is at the time more numerous than usual. The destruction 

 of a portion of the parasites may not only involve no loss 

 from an economic point of view, but may actually be a bene- 

 fit, in that it will extend the period of effective operation of 

 the parasite, and put off the time when it will cut off its own 

 food supply by its too rapid increase. The probabilities do 

 not justify the assumption that a bird usually does harm 

 rather than good in eating a parasite of an injurious phy- 

 tophagous insect. 



Nothing has been said in regard to those parasites upon 

 parasites which are called the secondary or hyper-parasites 

 of noxious insects. Our knowledge of the precise biological 

 relations of these is limited. On general principles it is prob- 

 able that when a bird eats one of these it is at least as likely 

 to be doing man a benefit as an injury. 1 



1 For an account of the relations between hymenopterous parasites 

 and their hosts, see Fiske, "The Parasites of the Tent Caterpillar," 

 New Hampshire College Agricultural Exp. Station, Technical Bulletin, 

 No. 6. 



