CLASSIFICATION OF BIRDS. 



375 



by incubation. The young are provided with a curious cal- 

 careous knob on the upper mandible, by means of which they 

 crack a hole in the egg-shell when ready to escape. Some can 

 feed as soon as they have escaped from the egg-shell — precocious 

 young ; others have to be fed by the parents — nestlings. 



Birds are found in all climates, and may be residents, 

 migrants, or gipsy-migrants. 



In cold and temperate climates many birds do not remain all 



Fig. 214. — Ovary of Bird. 



a, c, c, Ova in various stages of development ; b, streak without vessels indicating 

 the point -where the vesicle is about to rupture to allow ova to escape; d, ruptured 

 vesicle ; e, very small ovum showing cicatricula. (Chauveau.) 



the year; these are called "migrants.'' Migrants pass at the 

 approach of winter to a warmer climate, and return again to 

 nest in the spring. Prior to migrating the birds usuallj' collect 

 in flocks. These migrations generally take place about the same 

 date every year. Most migrants are insectivorous, and even if 

 the temperature were warm enough for them, they could not 

 subsist upon our scanty winter fauna. 



There are other birds whicli move from place to place in an 



