440 University of California Publications in Zoology L V L - n 



QUANTITY OF FOOD 



Three methods may be used in estimating the quantity of 

 food in a bird's stomach. First, the articles found may be 

 counted; second, they may be weighed; or third, they may be 

 estimated by volume. 



The first method, although important as giving an idea of 

 the bird's economic value by showing the number of injurious 

 insects or seeds destroyed, fails to take into account the difference 

 in size of the different articles, and does not show the relative 

 amounts of each kind. The second method has been generally 

 disregarded because of its impracticability. The third or vol- 

 umetric method allows of a balance of the inequalities of size, 

 and best portrays "the ratios each element bears to the others." 



By the numerical method, fifty ants would be placed against, 

 say, six ground-beetles. A computation made by the percentage- 

 by-volume method would doubtless show that these two kinds of 

 food represented only three and twenty-six per cent, respectively, 

 of the whole food. Hence the idea furnished in the first case 

 (a ratio of 50 to 6) is a misleading one. Numbers of one insect 

 cannot be balanced against the numbers of another insect. As 

 each bird of the same species has a certain average stomach 

 capacity, the ratio of each element to this average capacity gives 

 the most accurate idea of the relative proportions of each kind 

 of food. 



Although the first and last methods have been those most 

 often used heretofore, the second presents certain advantages 

 (e.g., mathematical accuracy) which should not be overlooked. 

 A combination of the numerical and the percentage-by-volume 

 method has been used in the present work. The number of birds 

 taking the different kinds of food offers further evidence as to 

 their capacity for good or ill. 



McAtee (1912) has pointed out that statements of numbers 

 of individuals in stomachs has an interest in direct proportion 

 to the bigness of the number. Believing this to be largely true, 

 the maximum number of individuals found in the stomachs has 

 here received emphasis. These maximum numbers should not 

 be considered as averages. 



