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 
modication 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 sternwm 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. 197). 
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 im 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 impossibilty of obtaining food when 
the cold of 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 
