284 GENERAL ZOOLOGY 



that a bird is bright colored is not necessarily to be taken 

 as indicating that it is conspicuous in its usual habitat. 

 Parrots, for example, are very difficult to see among the 

 foliage of tropical trees. In some instances, however, 

 striking colors, may serve "for attracting the opposite sex, 

 or as recognition marks for other members of the same 

 species. 



The nesting habits of birds show various degrees of com- 

 plexity and specialization in styles of architecture. A 

 nighthawk builds no nest, but deposits the eggs on the 

 bare ground, or sometimes even on a gravel roof. The 

 burrowing owl of the western prairies digs a hole and 

 strews manure on the bottom to keep the eggs off the 

 ground. The bush-turkey of Australia builds great mounds 

 of sticks and dead leaves in which the eggs are left to incu- 

 bate from the heat generated by the decaying rubbish. 

 Most passerine birds build nests like the robin, on the 

 ground or among the branches of plants. The apex of 

 avian architecture is reached in the beautiful basket nests 

 of the orioles. 



With a few exceptions birds incubate the eggs in nests or 

 cavities which they construct. In general the length of 

 the incubation period is proportional to the size of the 

 egg, the smallest hatching most quickly. The emperor 

 penguin carries its single egg between the hind legs, and 

 male and female birds take turns holding it. Eggs laid 

 in open nests are usually colored to match the surround- 

 ings, and this is particularly striking in those which are 

 laid on the ground without a nest or with but a very crude 

 one, as in the nighthawk or killdeer. Birds which nest 

 in hollow trees or in holes in the ground usually lay white 

 eggs. 



Domesticated birds play a considerable part in modern 

 civilization. The numerous varieties of the common hen 

 probably all come originally from the jungle-fowl, Gallus 

 gallus, of India. Domestic pigeons were derived from the 

 blue-rock pigeon, Columba livia, a native of Europe and 



