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 equally 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 
habits. Thus a single species or several allied species may become 
exceptionally abundant for a month or two in a very limited district 
devoted largely to a single crop on which 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 destroy annually 82,000,000 worth of the crop. 1 
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 whicli have beneficial food habits themselves destroy still 
more useful species, as exemplified by the cowbird when it parasitizes 
the song sparrow. The English sparrow, which does more good than 
harm to vegetation in the city 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 injury in the food habits can only be 
roughly 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 injury 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 Tiphia inornata, 
a species of wasp, it is doing an indirect harm, because this wasp 
parasitizes the larvae of May-beetles, which are exceedingly injurious 
to crops. So, too, when the chipping sparrow feeds on the cabbage 
worm (Pieris rapce) it is accomplishing an indirect good, because if 
the worms increased unduly they would destroy the whole cabbage 
patch. 
While the direct effects are easily observable, the indirect effects 
are usually obscured. Their complexity is frequently 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 organic 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. 247, 1887. 
