DESTROYERS OF RODENT PESTS 95 



taken into account, thus leaving only two injurious 

 hawks and no injurious owls that are at all common. 

 The chief harm done by the birds in this class is hi 

 destroying poultry and valuable insectivorous and 

 game birds. 



Thus approximately ten per cent of the species of 

 hawks and owls are harmful, fifteen per cent are 

 neutral, and seventy-five per cent are beneficial. 



Some bird-students place the pigeon hawk and 

 great horned owl in the harmful group instead of the 

 neutral group, the former on account of the number 

 of song-birds destroyed, and the latter on account of 

 the poultry and game-birds eaten. 



The following table is based on the results of a 

 study of hawks and owls made by the Bureau of 

 Biological Survey. In this study about twenty- 

 seven hundred stomachs were examined. The per- 

 centages here given are based on the number of 

 stomachs found to contain the different articles of 

 food. A more accurate estimate would be obtained 

 by estimating the percentage of the total amount of 

 food, but the table gives a fair estimate of the eco- 

 nomic status of each bird. In many cases the stom- 

 achs were found empty, and hi computing this table 

 the percentages have been based on the number of 

 stomachs found with food. By this method of com- 

 puting the percentages, the sums for one bird total 

 more than one hundred, because usually a bird had 

 fed on several kinds of food. 



