CLASS AVES. 



127 



annual molts, while with some birds there is a seasonal 

 plumage a summer and a winter with parti-colored va- 

 riations. 



The distribution of the plumage over the body in 

 tracts with most birds, permitting thereby ease of action 

 and smoothness of the feathers under every variety of 

 movement of the bird, is a remarkable feature, to which 

 may be added contrary overlapping of the feathers of 

 some certain sections, to likewise preserve smoothness 

 under even twisting movements. 



All birds are oviparous.* 



Fm. 219. 



* Incubation varies according to the species. The temperature needed is 

 104 Fahr. The chick first pierces its shell by a pegging motion of the hardened 

 point of the under mandible, and then enlarges the aperture by pressing with the 

 knob-like tip of the upper mandible. This knob falls off soon after the chick 

 emerges from its shell, as Nature abhors a useless appendage. Finally, by turn- 

 ing from left to right, it breaks the shell about half around, when a vigorous 

 stretch of the body, assisted by a bracing with the feet, parts the shell trans- 

 versely, and the young bird is liberated. Sometimes the fracture extends entirely 

 around the egg, and the two portions are completely separated from each other. 

 But if the inner membrane of the shell be not wholly divided, the connecting 

 portion serves as a hinge, and the two parts of the shell may, in the movements 

 of the young bird, become set, like two cups, one within the other. 



The following is taken from Baird, Brewer, and Ridgway, as a specimen of 

 what every thorough teacher will require from each of his pupils in writing, with 

 simple sketches attached, for every 

 bird that can be obtained in the im- 

 mediate vicinity : " Turdus migratorius 

 (tiir'dus mi gra to'ri us) [Robin], Ameri- 

 ' can Redbreast. Tail slightly rounded ; 

 above olive-gray ; top and sides of the 

 head black; chin and throat white, 

 streaked with black; eyelids and a 

 spot above the eye, anteriorly white ; 

 under parts and inside of the wings 

 chestnut-brown ; the under tail-coverts 

 and tibise white, showing the "plum- 

 beous inner portions of the feathers ; 

 wings dark brown ; the feathers all Sketch of principal parts of the Robin, 



edged more or less with pale ash ; tail 



still darker, the extreme feathers tipped with white ; bill yellow, dusky along the 

 ridge and at the tip. Length 9.75; wing 5.43; tail 4.75; tarsus 1.25 inches. 

 Habitat, whole of North America." 



