58 USEFUL BIRDS. 



While the Catbird's stomach iiiitrht not be laro-e enouo-h to 

 contain thirty full-grown locusts, it would easily contain more 

 than thirt}" small ones. The statement that thirt}" grasshop- 

 pers were found in the Catbird's stomach might also need 

 modification in another way. The least fragment of an in- 

 sect found in a bird's stomach is usually considered good 

 proof that the bird has eaten that insect. There might ])e 

 found in the stomach of a bird a mass of unrecognizable 

 material, from which the expert would be able to sort out 

 and recognize enough of the harder parts of different grass- 

 hoppers to prove that thirty of these insects, of consider- 

 able size, had been eaten within a certain time, even though 

 a greater part of those first swallowed had already disap- 

 peared from the stomach. 



Prof. r. E. L. Beal writes me as follows reo:ardino- the 

 methods used at the United States Department of Agri- 

 culture in countino; the insects found in the stomachs of 

 birds : — 



111 the case of grasshoppers and caterpillars it is the jaws (mandi- 

 bles) that are counted. Birds when not sleeping appear to eat all the 

 time when not occiiijied in other duties, such as nest-making or feeding 

 their young. The process of digestion is continuous. The more easily 

 digested parts pass out of the stomach very quickly, but the hard parts 

 remain somewhat longer. In this way when a bird is feeding upon 

 grasslioppers the jaws of those first eaten remain after the rest of 

 the l)ody has passed on. When the stomach is 02)ened the jaws are 

 comited, and for every two we estimate at least one grasshopper killed. 

 In cases where only a few insects were involved I have taken the pains 

 to pair the jaws, and in this way have often found that the number that 

 had been eaten was more than half the number of jaws. In this work 

 each head that appears to be whole is carefully examined, to see that it 

 has not lost one or more of its jaws ; were it not for this precaution, 

 the insect might be counted twice. Caterpillars, like grasshoppers, 

 are easily broken up, and so the heads are counted when whole ; other- 

 wise the jaws are counted. 



The variation in size of different species of insects should 

 also be considered. While the caterpillars of some species 

 of moths reach three or four inches in length, others never 

 grow to be half an inch long. 



These and other similar considerations, well known to 



