16 UNIVERSITY OF COLORADO. 



QUANTITIES OF FOOD REQUIRED BY BIRDS 



In order to fully comprehend the significance of food percentages in dte 

 cussing birds, it is quite necessary to understand that they require a much 

 larger amount of food in proportion to their weight than do any other 

 vertebrate animals, for several reasons, (a) Their temperature and rate 

 of respiration are much higher than in case of mammals, the only other 

 group of "warm-blooded" animals. A drop of a few degrees in the blood 

 temperature of a warm-blooded animal, except in case of hibernation, is 

 fatal. Food is the fuel which maintains that high temperature, (b) The 

 adults are vastly more active than other vertebrates. The prodigious en- 

 ergy displayed by the buzzing of the wings of a hummingbird as it hovers 

 over a flower, the great distances traversed in an evening by a nighthawk 

 as it circles in search of insects, the tremendous speed maintained by a teal 

 duck for long periods, grade through various species down to the 

 more moderate movements of the sparrows; but even the most sluggish is 

 very much more active than any member of any other group of vertebrates. 

 This is especially true when they are feeding their young, and so must 

 gather in some instances many times as much food as the parents them- 

 selves require. During this season their activity continues almost throughout 

 the day, from dawn until dark. Food furnishes the motive power for this 

 great and ceaseless activity, (c) What the nestlings lack in activity is more 

 than made up in rapidity of growth. The number of broods of young varies 

 with different species, but probably averages 2 or 3 broods of from 3 to 5 

 young each per annum, which require constant feeding. At first the nestlings 

 of most species consume more than their own weight of food in a day (with 

 most species chiefly soft-bodied insects), and gain in weight at the rate of 

 from 20% to 60% daily (28), usually reaching adult weight in from 4 to T> 

 weeks. Only large quantities of easily digested food could produce such a 

 rate of growth. 



Birds with crops fill both crops and stomachs with food about twice a 

 day. Probably insectivorous birds fill their stomachs on an average about 

 5 or 6 times a day (29). Hence the amount of food found in a bird's 

 stomach represents in one case about half, and in the other only one-fifth or 

 one-sixth of the food taken in a day. Experiments show that it requires 

 from 3 to 4 hours for young meadowlarks to digest grasshoppers, which 

 means at least 3 or 4 meals per day (30). On the other hand, of course, grain 

 is digested more slowly, the inactive, grain-eating poultry in confinement, re- 

 quiring from 12 to 24 hours (31). A shrike in captivity, when fed on May- 



<28) Judd, Sylvester D., The Food of Nestling Birds, U. S. Dept. Agric., Yearhook 

 for 1900, pp. 411, 435-436. Bergtold, W. H., A Study of the House Finch, The Auk, 

 Vol. XXX, pp. 64-68, 1913. Sherman, Althea R., At the Sign of the Northern Flicker, 

 The Wilson Bulletin, Nos. 72-73, Vol. XXII, pp. 169-171, 1910. 



(29) Bailey, Vernon, Birds Known to Eat the Boll Weevil. U. S. Dept. Agric., 

 Biol. Surv., Bull. No. 22, p. 15. 



(30) Bryant, Harold D., The Numbers of Insects Destroyed by Western Meadow- 

 larks, Science, n. s. Vol. XXXVI, p. 874, 1912. See also Univ. Call Pub in Zool 

 Vol. XI, p. 7. 



(31) Brown, E. W., Digestion Experiments with Poultry, U. S. Dept. Agric., Bureau 

 of Animal Industry, Bull. No. 56, pp. 77-78, 1904. 



