450 Vertebrata. 



It is homologous witli the copulatory organ of Chelonia and Crocodilia ; 

 it is situated on the ventral wall of the cloaca ; the free tip is directed 

 backwards, and is provided with a superficial groove, at the anterior 

 end of which the vas deferens opens, and along which the spermatozoa 

 pass during coitus. In Ducks the penis is spiral, in others, linguiform, 

 or rod-like ; the tip is usually invaginable. Where the cock-birds 

 have a penis, the hens usually have a rudimentary copulatory organ 

 (clitoris) . 



Very generally lai-ger or smaller external (secondary) sexual differences 

 are noticeable : usually the males are somewhat larger (in Fowls, etc.), rarely 

 smaller than the females (in Birds of Prey) ; the cocks are often distinguished 

 by the special development of certain feathers (Peacock, Birds of Paradise), by 

 peculiar cuticular processes (spur'of the Cock), or by vivid colouring. 



Most Birds breed only once a year (usually in the spring), others 

 several times {e.y., the House Sparrow). As a rule, they live in 

 pairs during the breeding season, i.e., they are monogamous; 

 occasionally the male has several hens, or is polygamous. 



The eggs of Birds are of very considerable size, and contain a 

 large amount of food yolk. As they pass down the oviducts they are 

 covered first by a mass of albumen, then by a shell-membrane, and 

 finally, in the uterus, by a hard calcareous shell ; all the coverings 

 are secreted by glands in the wall of the oviduct. The eggs are 

 incubated by the females alone, or by the males and females 

 together, rarely by the males only, this occurs in the African Ostrich 

 and the Phalarope [Phalaropun) ; usually the sitting Bird is provided 

 with brood spots, regions from which the feathers have 

 fallen off, so that the eggs may come into direct contact with the 

 warm skin. Most Birds build nests for the reception of their eggs, 

 but occasionally they are laid upon the bare ground. In the simplest 

 cases the Birds drag together a scanty collection of twigs, straws, 

 feathers, etc. ; in others similar materials are woven into a basket- 

 shaped or spherical nest; occasionally the nest is built of clay, dirt, or 

 the like, and saliva (Swallows and others), or from saliva alone 

 (Salangane). The nests of some forms are built upon the ground, 

 and others in excavations or in natural holes in the earth (Sand- 

 martins, Pufiins), in holes in trees (Woodpecker), on trees, etc. 

 Usually the males and females build the nest together. As a rule the 

 young Birds do not leave the nest immediately after hatching, but 

 remain in it for some time, and are fed by the parents ("altrices ") ; 

 occasionally ("prsecoces ") they are immediately able to feed them- 

 selves (usually, however, under the protection of the hen). The 

 newly-hatched young one generally differs considerably from the 

 adult ; it is either covered with down or is almost naked, and differs 

 in colour, and usually in the form of the beak {e.g., in many singing 

 Birds) ; the food, too, is often different from that of the adult (for 

 instance, many grain-eating Birds feed their young ones upon Insects). 



