AVES 



613 



well developed for bipedal locomotion and perching. The muscles 

 of the long flexible neck are quite intricate. Since this animal does 

 no chewing, the jaw muscles are not strongly developed. The large 

 pectoral muscles which extend from the sternum to the upper arm 

 (Fig. 327) constitute from 15 to 20 per cent of the weight of the body 

 and make up the breast meat of the bird. The muscles of the thigh and 

 shank or ''drumstick" are quite well developed, and the chicken is a 

 strong runner (Fig. 326). Certain muscles and ligaments of the legs 

 are arranged in such a way that the bird is able to cling to the perch 

 due to the down pull of the weight of the body. This makes it 

 possible for the chicken to sleep on the perch without danger of 

 falling off. 



LEFT TESTIS 



LEFT VAS DEFERENS 



CLOACX 



PAP;'LLA OF VAS D. 



Fig. 328. — Reproductive organs of male chicken. (Drawn by Titus Evans.) 



Reproduction and Life History 



The sexes are distinct but there is no definite pairing for repro- 

 duction; one cock may mate with several hens. The eggs are 

 usually laid in a rounded nest constructed of hay or other soft 

 material. Some birds build no nest at all, Avhile others build very 

 elaborate ones, such as the hanging model of the Baltimore oriole. 

 Copulation is necessary for the transfer of mature spermatozoa from 



