2 V. VERTEBRA TA : A VES. 611 



etc.) well developed. The beginnings of an external ear are seen in 

 the deeper position of the drum membrane. Since the power of 

 flight necessitates vision at great distances, most birds have exceed- 

 ingly sharp sight, and the eye itself (fig. 642) is in general con- 

 re 



Op 



FIG. 642. Eye of owl. (From Wiedersheim.) Oi, choroid; CM, ciliary muscle; Co, 

 cornea; Cv, vitreous body ; Ir, iris : L, lens; Op, optic nerve; OS, sheath of nerve ; 

 P, pec ten; Rt, retina; Sc, sclera; VK, anterior chamber; t, sclerotic bones. 



structed for distance. Peculiarities of the bird's eye, already 

 weakly developed in the reptiles, are the pecten (P), a comb- 

 shaped growth of the choroid into the vitreous body, and the 

 scleral ring, a circle of bones developed in the sclera and support- 

 ing the outer part of the eye. 



Among birds there is spirited rivalry for the females, especially 

 among polygamous species. At the time of mating the males seek to win 

 the favor of the females either through striking motions (dances), by 

 singing, or by beauty of plumage. All of these peculiarities are confined 

 to the male and frequently lead to a marked sexual dimorphism. The dis- 

 tinction in plumage is commonly strengthened at this time, the male 

 receiving the brilliant wedding dress. Thus we speak of the spring molt, 

 although there is only a color change and only exceptionally a renewal of 

 the feathers. The return to every-day clothes only occurs with a molt, and 

 this comes at the close of the reproductive season. 



The reason for the dull plumage of the female is due to the fact that 

 she usually sets on the nest, at which time inconspicuous colors protect her 

 from destruction by enemies. In only a few instances is the heat neces- 

 sary for incubation produced by other causes, such as the heat of the sun 

 upon the sand in which the eggs are buried, or the increase of temperature 

 caused by fermentation in decaying vegetation (Megapodes). The rule is 



