460 McAtee, Contents of Bird Stomachs. [oct. 



the Black-head, consisting almost wholly of injurious insects, is 

 practically twice the bulk of the vegetable food, or more than four 

 times that portion which is pilfered from man"; and that "for 

 every quart of fruit eaten, more than 3 pints of black olive scales 

 and more than a quart of flower-beetles, besides a generous sprink- 

 ling of codling moth pupse and cankerworms fall prey to this gros- 

 beak." 



The percentage-by-bulk system has a further advantage in that 

 it indicates approximately the proportion of the total feeding time 

 a bird spends in eating the various elements of its food. For 

 instance, if we state that a certain Blackbird spends about half 

 its time eating grain, as we may with approximate truth if 50 

 per cent of its food is grain, we present this fact in a much more 

 graphic way, than we would be able to do, were our knowledge 

 confined to the number of kernels contained in a series of stomachs. 

 The writer expressed this idea in a slightly different form when 

 writing about the food of wild ducks. He then said: ^ "Although 

 on first thought a percentage of less than 5 for wild rice may seem 

 small it really means that these 16 species of ducks get a twentieth 

 of their annual subsistence from this grain; in other words, the 

 quantity they eat would support them for two and a half weeks, 

 if wild rice were fed upon exclusively. Similarly, wild celery, which 

 forms 6.65 per cent of their food, would suffice for three and a 

 half weeks; and pondweeds, which form 13.88 per cent, for more 

 than seven weeks." 



These illuminating expressions of the importance of various 

 items of bird food are impossible under any numerical system. 

 Neither does a numerical system supply the basis for graphic 

 representation of the proportions of bird food, such as the sectors 

 of circles method devised by Judd,* the curve plottings introduced 

 by Professor Beal,^ or the shaded columns used by the writer.* 



The chief beauty of the percentage system however is that it 

 permits those comparisons of one part of a bird's diet with another 

 part, or the food of one species or group of species with that of 



» Circular 81, Biol. Survey, 1911, p. 2. 



» See Yearbook U. S. Dep't of Agriculture, 1900. 



• See Bull. 13, Biol. Survey. 



* See Bull. 23, Blol. Survey, 1905, p. 29. 



