20 USEFUL BIRDS. 



birds eat a great many caterpillars containing parasites, 

 though birds will reject any caterpillars that show signs of 

 weakness or disease. The question then arises, Is the bird 

 doing harm by eating caterpillars or other larvse containing 

 parasites? The bird certainly ends the destructiv^e career 

 of the larva at once. The parasites would have ended it 

 eventually ; but had it been left to them, it might have gone 

 on for some time in its destructive career, doing as much 

 injury as if not parasitized ; the parasite merely destroys it 

 in time to prevent it from propagating its kind. So far the 

 evidence is in favor of the bird. The question remains, 

 however, whether the bird and its young would eventually 

 destroy more caterpillars than would the progeny of the 

 parasites had the}" not been eaten by the bird. This question 

 evidently is unanswerable. Birds act as the primary check 

 on the increase of destructive insects ; parasitic insects are 

 the secondar}" check provided by nature to operate in con- 

 junction with the l)irds, or to supplement the regulative 

 action of birds where the number of birds is insufficient to 

 check the increase of insects. 



Birds sometimes kill many of the imagoes of parasitic 

 insects in flight, where such insects are numerous. At first 

 sight, this would seem to condemn the birds ; on further 

 study, it seems probable that this is often a harmless habit. 

 Where parasitic insects are found in great numbers, it is 

 probable that the birds destroy mainly the surplus flies, 

 which otherwise, failing to find hosts for their 3'oung, would 

 merely live out their time and die without issue were they 

 liot killed b}' the birds. Such harm as birds do in killing 

 primary parasites may be ofiset by the killing of secondary 

 parasites by birds, for this acts as a protection to the pri- 

 mary parasites. 



Certain predaceous bugs feed not only on insects but also 

 on vegetable food. They also attack other predaceous or 

 useful insects. Birds, by preventing their undue increase, 

 may prevent excessive injury to both useful plants and 

 insects. 



All reasoning from known premises leads to one conclusion 



