12 NEW YORK STATE MUSEUM 



looked equally attractive to the casual observer had no warblers among 

 their branches. These illustrations easily indicate the relationship between 

 food and the abundance of a species, but it must be borne in mind that 

 similar relationships are all the while at work governing the abundance 

 of birds when the exact cause is not manifest to the observer. 



As allies, some animals afEect slightly the abundance of species. 

 Undoubtedly the Kingbird assists the Yellow warbler and other birds 

 in escaping from the depredations of hawks and crows when nesting in 

 the same orchard, and the various little associations observed, such as 

 the Downy woodpecker, Nuthatch, Chicadee, Creeper and Kinglet coterie, 

 have a real cause for their existence other than the desire for mutual 

 companionship . 



The most evident cause of the disappearance of birds, to the casual 

 observer, is the enemy factor. Under the head of enemies must be classed 

 all beasts and birds of prey, rivals and parasites. To illustrate the influence 

 of these factors, it is often observed by bird students that a single pair 

 of Sharp-shinned hawks will destroy nearly every song bird in the wood 

 where they are nesting, and a cat which has discovered a brood of bob- 

 whites will return to their range and follow the birds until every chick 

 has been destroyed. In the same way the cat destroys the broods of 

 numerous birds which nest in the garden and dooryard, and the parasitic 

 Cowbird which lays her egg in the nest of a small warbler or sparrow 

 thereby destroys the entire brood of the other bird. To illustrate the 

 influence of rivalry, we might mention the effect of the English sparrow 

 upon the martins and bluebirds. By continually occupying their nesting 

 sites it is gradually forcing these birds more and more from our dooryards, 

 and, unless the martins and bluebirds are assisted, they will finally be 

 driven from the immediate vicinity of our homes. I have also observed 

 that the Wren frequently picks holes in the Bluebird's eggs, and thus 

 destroys one of her rivals in the race for food. The problem of food rivalry 

 is rather complex, but unquestionably is very often of a determining 

 influence in governing the abundance of various species. Parasites must 



