1912 J McAtee, Contents of Bird Stomachs. 461 



another, which are a siiie qua non in scientific economics. It means 

 so little to say for instance as Mason does: "Of 110 insects taken 

 by 35 birds [Common Mynah], 58 are injurious, 5 beneficial and 47 

 neutral," (p. 103) and "of 39 insects taken by 14 birds [Pied 

 Mynah], 1 is beneficial, 25 injurious, and 13 neutral." (p. 109). 

 How can the import of these figures be judged unless they are put 

 into proportions? And they cannot be so compared, from the 

 data given, since the insects are of many sizes, and consequently 

 of varying economic importance. 



But when we read that weed seeds form 36 per cent of the annual 

 food of the Cardinal and 15 per cent of the diet of the Rose-breasted 

 Grosbeak, we can appreciate at once the comparative value of weed 

 seed as food to these two birds, and the rank of the species as 

 destroyers of weed seeds. Citation of long lists of the numbers 

 of neutral, beneficial and injurious insects which are not susceptible 

 of direct comparison, soon confuses the mind, while the same facts 

 expressed in percentages have directness and clearness obtainable 

 in no other way. 



Professor S. A. Forbes, the pioneer economic ornithologist, 

 whose skillful laboratory work and clear thinking, laid so firm a 

 foundation for subsequent workers in this field, adopted the 

 percentage-by-bulk method. He explains ^ that in stomach exami- 

 nation "opportunity is afforded for careful and trustworthy esti- 

 mates of the ratios each element bears to the other, so that the 

 average significance of the food can be discovered. Practically, 

 this is indispensable. Whatever method fails of this, while its 

 results may be interesting, and may have a certain general value, 

 can never afford a basis for anything better than indefinite opinion. 

 It can never settle the case for or against the birds. 



" This method, while by far the best of the three, has its slight 

 disadvantages. Some things eaten by birds leave no appreciable 

 trace in the stomach. For example, it is difficult, by this method, 

 to determine with certainty those birds which greatly injure grapes 

 by breaking the skin of the fruit and sipping the juice. This 

 difficulty applies only to liquid food. Other errors may arise 

 from the shorter or longer periods for which different kinds of food 

 will last in the stomach; but of this we have no proof. I have 



1 BuU. lU. state Lab. Nat. Hist.. Vol. I, No. 3, 1880. 



