42 USEFUL BIRDS. 



fluids ; the chyle is drawn off by the lacteals, and the residue 

 is excreted. The vigor, perfection, and rapidity of these 

 processes in insect-eating birds are such as might be expected 

 among animals of such high temperature, perfect respiration, 

 and rapid circulation. 



The various dilations of the digestive tract serve well their 

 purpose of enabling the bird to consume the large amount 

 of food necessary for its maintenance. Digestion is partic- 

 ularly rapid in the growing young of most birds, for they 

 require not only food sufficient to sustain life, but an extra 

 supply as well to enable them to increase daily in size, and 

 to grow, in a few days, those wonderful appendages that we 

 call feathers. 



The Growth of Young Birds. 



The growth of many birds from the egg to the hour of 

 flight requires less time than is needed by some insects to 

 reach the flight stage. It is most significant that young birds 

 can develop as rapidly as can many in- 

 sects on which they feed, for it shows how 

 readily, under favorable conditions, the 

 increase of birds might keep proportion- 

 ate pace with that of insects. Weed and 

 Dearborn, in their interesting manual, en- 



titled " Birds in their Relations to ^n," 

 naked, blind, and help- state that they watched four young Sons: 



less, with mouth open o , , ' ,. , , 



for food. Reduced; Sparrows that were out of the nest on the 

 after iierrk-k. eighth day. Mr. Owen records another 



instance where a brood of young Song Sparrows were 

 fledged and left the nest within the same period. 1 Probably 

 this is exceptional ; but many of the smaller birds rear their 

 young from the egg to the first flight within two or three 

 weeks. Mr. Owen found that on one particular day this 

 family of five young Song Sparrows increased in average 

 weight forty-eight per cent., while the smallest bird gained 

 fifty-five per cent, in a single day. 



The young of perching birds (Insessores) come into the 

 world tiny creatures, either naked or covered Avith down, 



1 A Family of Nestlings, by D. E. Owen. The Auk, Vol. XVI, No. 3, July, 

 1899, pp. 221-225. 



