86 FOOD HABITS OF THE GBOSBEAKS. 



RELATIONS OF GROSBEAKS AND OTHER BIRDS TO PARASITIC 



INSECTS. 



As in the previous pages mention is made of the fact that gros- 

 beaks to some extent feed on parasitic insects, and as many other 

 birds have the same habit, certain phases of the subject are here 

 briefly discussed. Parasites are useful to man because they prey 

 upon and reduce the numbers of injurious insects. Hence to the 

 extent that birds diminish the number of parasites they are inimical 

 to man's interests. But there is another aspect to the interrelations 

 of birds, parasites, and injurious insects. The attaclvs of parasitic in- 

 sects, however numerous they may be, do not result in the immediate 

 death of their victims, since time is required for the development of 

 the larvae within or upon the body of the hosts. Hence the latter, 

 after receiving the eggs which ultimately are to prove fatal, some- 

 times continue their depredations, and may in a few days ruin 

 the season's crop. Under these circumstances the parasites do no 

 immediate good, though they serve to prevent increase of the species 

 attacked. It is quite otherwise with birds, which instantly stop 

 de]3redations by killing the insects responsible for them. Often in 

 a few days birds extirpate a pest over a limited area, and the crop, 

 freed from its enemies, makes successful growth. It is true that at 

 the same time all eggs and larvse of parasites present in the host 

 insects also are destroyed. Though this is regrettable, the effects 

 of the loss of the parasites must be regarded as of secondary' im- 

 portance, since the main purpose — getting rid of the pests — is accom- 

 plished. 



Precisely the same result follows the destruction of cocoons, the 

 burning of caterpillar nests, fumigation by hydrocyanic gas (which 

 is exceedingly destructive to all life) , and wholesale killing by contact 

 sprays (kerosene, whale-oil soaps, and the like). These methods of 

 destroying insects are widely recommended and extensively used, re- 

 sulting in the indiscriminate destruction of beneficial and injurious 

 insects, foe and friend alike. Moreover, there is no doubt that in 

 spite of their indiscriminate effect, these practices result in great good. 

 As stated above, birds are chargeable with similar destruction of use- 

 ful insects along with the injurious, but it is worth}^ of note that the 

 effects of their feeding have not been judged from the same stand- 

 point nor given the benefit of doubt accorded to the work of artificial 

 agencies. However, since it has been suggested that birds may so re- 

 duce the number of parasitic insects as seriously to restrict their bene- 

 ficial services, it may be profitable to inquire into the facts of the case. 

 There is no record of an instance in which noticeable injury has been 

 caused through the destruction of parasites by birds, and there is 

 much evidence going to show that this never happens. First, taking 



