148 BIRD GALLERY. 



The intestinal canal terminates in a cloaca or cavity through which 

 the products of the urino-genital organs also pass. 



Trachea or Windpipe. Finally as an important anatomical peculiarity 

 of this class of Vertebrates, we have to mention that the trachea or 

 windpipe is composed of a series of entire osseous rings. The organ 

 of voice is not the larynx as in the mammals, but is formed by a peculiar 

 medication of the lower end of the windpipe called the syrinx. The 

 syrinx may be formed either by the trachea or by the bronchi only, but 

 most commonly the lowest rings of the trachea as well as the bronchi 

 participate in its formation. The modifications of the voice or song of 

 a bird are regulated by a pair of " extrinsic " and, in the Song- 

 Birds, several pairs of <( intrinsic " muscles. The former, possessed 

 by all birds, generally pass from the trachea to the sternum and 

 furcula. The intrinsic muscles may be absent or represented by five 

 or seven pairs. These differences afford important characters for the 

 purpose of classifying certain orders of Birds (cf. p. 107). 



Nest and Eggs. Birds are, without exception, oviparous. The 

 majority deposit their eggs in a nest which they specially prepare for 

 their reception. Incubation lasts for a shorter or longer period and 

 varies from 11 to 56 days. 



The eggs are on the whole fewer in number than is the case in 

 Reptiles ; they possess a large amount of yolk and are invested with 

 a hard porous calcareous shell. 



Young. The young when hatched differ greatly in the relative 

 degree of development which they have attained. In the most primi- 

 tive condition the nestling emerges from the shell clothed in down and 

 capable of considerable activity, but in the most specialised it is blind, 

 naked and helpless when hatched, and requires to be fed and cared for 

 by its parents for some time. 



Mental Faculties. With regard to their mental faculties, Birds as a 

 class seem to occupy a position intermediate between Mammals and 

 Reptiles. Intelligence of a high order manifests itself in their social 

 relations with one another and in their various methods of obtaining 

 food. These faculties are still more developed in individuals which 

 come in contact or live with man. 



Migration. The difficulty or impossibility of obtaining food when 

 the cold ot winter destroys insect-life, or snow hides seed or other 

 vegetable nourishment, compels most birds to leave the locality where 

 they breed. Those which are stationary or range over only a limited 

 extent of country in search of food are termed resident birds. Their 

 movements are of an uncertain, erratic nature, and depend on external 

 and atmospheric conditions. But others, as soon as food becomes 

 scarce, following a common impulse, migrate at fixed times and by 



