The Food of Birds. 
151 
In order to determine the number of specimens which 
it is necessary to examine in each month, to reach reli- 
able averages of benefit and injury, I divided my notes on 
twenty of the specimens for March into two groups of ten 
each, so selected that all the localities and all parts of the 
month were equally represented in each group ; and then 
averaged each ten separately and compared the aver- 
ages. In the first group beneficial insects composed 
twenty-nine per cent of the food, and injurious insects 
fifty-nine per cent. ; in the second group beneficial insects 
composed twenty-seven per cent, of the food and injuri- 
ous insects sixty-one per cent. The close correspondence 
of these averages shows that, on this question, ten speci- 
mens would have given as accurate information as twen- 
ty, and indicates that ten birds a month will usually af- 
ford a fair basis for an opinion. 
April. 
The food for April, as shown by the thirteen specimens 
of that month (from Normal, Evanston, Waukegan, and 
Elizabeth, in 1876 and 1880), was remarkable for the num- 
ber of Aphodii (dung-beetles)- it included; twenty-one per 
cent, of the food of the month was Aphodius inqninatns, 
nine per cent. A. fimetarius, and one per cent, undeter- 
mined Aphodii. This peculiarity is accounted for, in 
harmony with what has been said above respecting the 
feeding habits of the bluebird, by the fact that this is the 
month when the Aphodii fly most actively in the latitude 
of northern Illinois. Carabidse now stand at eight per 
cent., including Carahus palusiris, Pterosticlius, Evar- 
thrus, and other Pterostichi, Platynus, Chlcemus tomen- 
tosus, Anisodactyliis rusticus, Amphasia inter si it i alls , 
and Harpalus : four per cent, of Hemiptera includes Cor- 
iscus and Hymenarcys nervosa, while spiders rise to nine 
per cent. Caterpillars are twenty-one per cent, (seven- 
teen per cent. Noctuids), June-beetles (Phyllophaga) two 
per cent., Curculionidse one per cent., and grasshoppers 
(Tettigidea sp. and Tettix ornata) eight per cent.; a 
total of thirty-two per cent, of injurious insects against 
twenty-one per cent of predaceous species. Among the 
