^°\f^^^] McAtee, Contents of Bird Stomachs. 457 



second in importance, " never composed more than one-fifth of the 

 entire contents [of stomachs], and usually less than one-tenth." 



It should be pointed out also that the numerical system would 

 be of no use at all in the case of a majority of mammal stomachs 

 as the food is so finely ground. Comminuted food presents no 

 great obstacles to percentage estimation, however, as nearly all of 

 its can be reckoned in the account by this system. Lord Kelvin 

 has said "When you can measure what you are speaking about, 

 and express it in numbers, you know something about it; but 

 when you cannot measure it, when you cannot express it in numbers, 

 your knowledge is of a meager and unsatisfactory kind; it may be 

 the beginning of knowledge, but you have scarcely in your thoughts 

 advanced to the stage of science." It follows therefore that a 

 method of estimating bird food which is powerless to express any- 

 thing about a considerable portion of the food, has "scarcely 

 advanced to the stage of science." 



Under the numerical system, the tendency is for insects or other 

 food elements with very resistant parts to get an undue representa- 

 tion among the items contained in the stomachs. For instance the 

 mandibles of grasshoppers or of beetles or certain stony seeds may 

 persist in the stomach while one or more meals following that from 

 which they are derived, have been eaten and digested. When 

 numbers only are used we must count one insect for each pair of 

 mandibles, and it often happens that the numerical majority 

 of the insects in a stomach, form but a small proportion of the food. 

 Under the percentage-by-bulk system the error due to the presence 

 of relics of past meals, is reduced to the minimum.^ 



The reason is that in examining large series of stomachs inequali- 

 ties of size tend to balance each other. The insects being entire, 

 a grasshopper will equal in bulk say 10 of a certain species of 

 carabid beetle, but another stomach may contain the jaws of 20 

 or more grasshoppers and only one of the carabids, which, however, 



1 It should not be understood from this and following remarks, that the writer 

 believes in the long persistence of food particles in birds' stomachs, for his position: 

 is just the reverse. What is meant is that in the great majority of birds, digestion 

 is a continuous process, and what may be termed a meal, i. ?. a stomach full of 

 freshly taken food, is generally accompanied by the harder portions of one, or 

 perhaps more than one previous "meal." It is probable that only in rare cases 

 Is any particle of food retained in a stomach more than a few hours. 



