348 



PRACTICAL ZOOLOGY 



begin to appear, and only a few spots of down remain to show 

 where the baby clothes still show through the contour feathers. 

 The first plumage is worn only a short time ; then it gives way 

 to a second plumage. The loss of one set of clothing and the 

 acquirement of another is called "molting." In adult birds 

 molting is annual or semi-annual. All birds shed their feathers 

 in the autumn, after they have finished their household duties 



for the season, and they 



put on their heavy woolen 

 winter clothing in theshape 

 of a beautiful new set of 

 plumage. In the spring 

 many birds change their 

 clothes again, and at this 

 time acquire the gorgeous 

 ornaments that are every- 

 where noticeable just be- 

 fore the breeding season, 

 such as the elegant plumes 

 of the snowy heron, known 

 as "aigrettes" (Fig. 261). 

 Internal Organs. — Cer- 

 tain peculiarities in the in- 

 ternal organs of birds may 

 be pointed out here (Fig. 

 217). The food is not 

 masticated, as there are no 

 teeth present . It is stored 

 in an enlargement of the oesophagus, the crop, where it is 

 macerated. In the stomach it is acted upon by digestive juices 

 from a glandular portion and ground up in the muscular gizzard. 

 Frequently small stones are swallowed to aid in grinding up the 

 food. 



The heart is comparatively large, and instead of a ventricle 

 partly divided in two, as in reptiles, there are two entirely sepa- 



FlG. 227. — Downy woodpecker at en- 

 trance to nest-hole in a dead poplar tree. 

 Her bill is filled with insects which she has 

 captured on near-by trees and is about to 

 feed to her young within the hole. (Photo. 

 by Hegner.) 



