628 COLLEGE ZOOLOGY 



Figure 509 shows diagrammatically the food of nestling and 

 adult house wrens, birds that are very common about gardens. 

 Practically all of the insects devoured by birds are injurious to 

 plants or animals and consequently harmful to man. 



Another large element in the food of birds consists of small 

 mammals, such as field-mice, ground-squirrels, and rabbits. 

 For many years hawks, owls, and other birds of prey have been 

 killed whenever possible, because they were supposed to be in- 

 jurious on account of the poultry and game-birds they captured. 



FIG. 509. Diagram showing the kind and comparative quantity of ' food 

 of the nestling (A) and adult (B) house wren. (From Judd, Bui. 17, Bur. Biol. 

 Survey, U. S. Dep't Agric.) 



Careful investigations by Dr. A.. K. Fisher have shown, however, 

 that at least six species are entirely beneficial; that the majority 

 (over thirty species) are chiefly beneficial; that seven species 

 are as beneficial as they are harmful; and that only the gyrfal- 

 cons, duck-hawk, sharp-shinned hawk, Cooper's hawk (Fig. 495), 

 and goshawk are harmful. 



As examples of beneficial birds of prey may be mentioned 

 (i) the rough-leg hawk, which feeds almost entirely on meadow 

 mice during its six months' sojourn in the United States, (2) the 

 red-tailed hawk, or " hen hawk," sixty-six per cent of whose 

 food consists of injurious mammals and only seven per cent of 



