1920.] 



The Rook. 



869 



Apart from the fact that long lists of the names of insects, 

 seeds and other foods eaten by birds are very difficult to com- 

 pare one with another, the results are open to various interpre- 

 tations. Supposing that we know that a certain bird has eaten 

 50 injurious and 25 beneficial insects, we learn nothing from 

 these figures as to the ratio one lot of food holds to the other. 

 Again, the apparent ratio of 50 to 25 is inaccurate, for the 

 former may be composed of cockchafers and the latter of 

 ichneumon flies. If, however, we can state the percentage of 

 bulk that these two kinds of insects hold, then we have ascer- 

 tained a definite piece of valuable information, capable of 

 being used for purposes of comparison. 



It must be borne in mind that a bird does not require so 

 many injurious insects, so many beneficial insects and so many 

 seeds per day, but a certain bulk. of food, generally speaking 

 three to four times the cubic capacity of its stomach; and if 

 we have to form an estimate of the economic importance of 

 this or that item constituting such bulk, it is essential that we 

 should first know what proportion the particular food item 

 constitutes to the birds' daily requirements. In order to 

 do so we must express ourselves in terms of some method of 

 measurement. 



Another very important point to remember is that by the 

 numerical method only part of the food is recorded, whereas 

 it is necessary that every bit of the food taken into the crop and 

 stomach be accounted for. 



Let us suppose that the stomach of a blackbird is found to 

 contain a mass of fruit pulp. How can we express this in 

 numbers according to the numerical method? Who can say 

 how many strawberries, blackcurrants, or plums are contained 

 in the mass? To do so is impossible, but the quantity can be 

 expressed in terms of its percentage, or the ratio it bears to 

 the remaining food items, and if we have a sufficient number 

 of stomachs, procured during each month of the year, then 

 we can state very definitely that the amount of fruit found 

 in the stomach of this bird averages, say, 14 per cent. In other 

 words, we would know that this particular species could have 

 eaten more fruit, but that it preferred other food, and also 

 that of the, say, 7 lb. of solid food that each bird of this 

 species requires in a year, 12 J oz. consist of fruit. If the per- 

 centage which this figure represents is compared with that of 

 injurious insects and other items, we can quickly form a rough 



V 



