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 the shape 

 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- 



FIG. 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.) 



