CLASSIFICATION OF BIRD FOOD. 17 



worthy of protection when the neutral part of its food forms less than 

 half of its entire food and its beneficial food amounts to several times 

 its injurious food. The native sparrows, it may be added, seem to 

 satisfy these conditions better than any other equallj^ large group of 

 birds. 



Exceptional habits must also be considered in determining a bird's 

 value, for they sometimes overshadow in importance the general food 

 liabits. Thus a single species or several allied species may become 

 excei3tionally abundant for a month or two in a very limited district 

 devoted largely to a single crop on Avhich they feed. An illustration 

 of this is the autumnal migration of bobolinks and red- winged black- 

 birds when the birds converge and swarm into the limited area of the 

 rice districts so as to destroj^ annually $2,000,000 worth of the crop.^ 

 Some species of birds act as agents in the distribution of the seeds 

 of noxious plants, as in the case of the crow, which is in a measure 

 responsible for the widespread distribution of poison ivy. Certain 

 species which have beneficial food habits themselves destroy still 

 more useful species, as exemplified bj^the cowbird when it parasitizes 

 the song sparrow. The English sparrow, which does more good than 

 harm to vegetation in the citj^park (though it has objectionable food 

 habits in rural districts), overbalances this good and becomes a pest 

 because of its filthy habits. 



But it is not easy to determine the exact relation of birds to agri- 

 culture, even though all the constituents of the food are known; for 

 the actual ratio of benefit to injurj^ in the food habits can onlj^ be 

 rouglily approximated, and it is often a question of nice judgment to 

 determine the final status of a particular species. The* benefit is 

 usually, if not invariably, indirect, while the injurj^ may be either 

 direct or indirect. When the English sparrow steals food from a flock 

 of chickens the harm is direct; but when it preys on Tipliia inornata, 

 a species of wasp, it is doing an indirect harm, because this wasp 

 parasitizes the larvae of Maj'-beetles, which are exceedingly injurious 

 to crops. So, too, when the chipping sparrow feeds on -the cabbage 

 worm {Pieris rcqjce) it is accomplishing an indirect good, because if 

 the worms increased unduly they would destroy the whole cabbage 

 patch. 



While the direct effects are easih^ observable, the indirect effects 

 are usually obscured. Their complexity is frequentlj^ baffling to the 

 investigator who is in search of economic conclusions. One is 

 brought face to face with most perplexing problems resulting from 

 the interaction of organisms — problems which not only embrace the 

 complex interrelations among animals and plants, but also include 

 the relations of oj-ganic life to its inorganic environment. The draw- 

 ing of sound economic conclusions is impossible until the far-reaching 

 influence of this interaction is at least thoroughly appreciated if not 



Ann. Report Dept. of Agr., for 1886, p. 24'i 



