Aves. 219 



egg," or albumen. The word " albumen " (from album, 

 white) now stands for the class of substances having the 

 same essential composition as the clear part of the egg. 

 After the albumen is added to the yolk, the oviduct secretes 

 a limy shell which completes the egg. The eggs pass from 

 the oviduct through the cloaca on their way out. 



Incubation. Most birds incubate the eggs. They are 

 sometimes laid on the bare ground, but generally in a more 

 or less carefully prepared nest. These nests vary from a 

 simple platform of sticks to the elaborate hanging nest of 

 the oriole, woven of grass and soft fibers, or the still more 

 ingenious nest of the tailor bird. It is to be noted that the 

 nest is " simply a cradle and not a home." 



The germ spot always turns uppermost, so that the 

 developing embryo gets the heat from the body of the 

 incubating bird. During incubation there is an increase 

 in the amount of blood circulating in the area in contact 

 with the eggs, a provision for affording heat to them. 



The Colors of Birds. The colors of feathers are due to 

 two factors, first to certain coloring matters, or pigments, 

 and second to the structure of the feather, most of the 

 luster and iridescence being due to the latter. As a class 

 birds are highly ornamented. The males are usually more 

 highly ornamented than the females. Perhaps this is 

 because the females are safer, during incubation, with dull 

 colors. But in many families the sexes are colored alike. 

 The young generally resemble the adult female. 



Molting. Birds shed their feathers and renew them at 

 least once a year. If the molting takes place but once a 

 year, it is usually in the fall, but some birds also renew 

 their plumage in the spring. In rare cases there is a third 

 molt. Most birds shed their wing quills in pairs, succes- 

 sively, so that they are at no time deprived of the use of 



