AVES 363 



forward at the point where the trachea divides. Muscles control 

 the tension of this valve and the number of vibrations and pitch 

 resultant are thereby regulated. 



Bird Songs. — Call notes are heard throughout the year, but 

 songs, which Darwin believed to be associated with sexual selection, 

 are limited in most birds to the breeding season. (See page 515, 

 Sexual Selection.) The rhea and the ostrich are said to be mute. 



References on Bird Songs 



Allan, F. H. Some little known songs of common birds. Nat. Hist., 



vol. 22, p. 235. 

 Oldys, Henry. 191 7. The meaning of bird music. American Museum 



Journal, vol. 17, February, pp. 123-126. 

 Saunders, Aretas A. Flight songs and mating songs. Auk, vol. 39, 



p. 172. 

 Tyler, W. M. 1923. Courting orioles and blackbirds from the female 



bird's eyeview. Auk, vol. 40, Oct., pp. 696-697. 



Excretory System. — The kidneys are paired and ordinarily three 

 lobed. The first lobe is usually larger although in the pelican the 

 third lobe is larger. The tern has 7 or 8 lobes in its kidneys and the 

 eagle has four. No urinary bladder is present. A rudimentary 

 urinary bladder is found most highly developed in the ostrich, while 

 the owl, pelican, grebe and swan have small ones. The urine 

 solidifies on reaching the air.'' The adrenals (suprarenals), endo- 

 secretory glands regulating blood pressure, are rounded and yellow- 

 ish and found on the inner edge of the first lobe of the kidney. 



Reproductive System. Male. — In the male, there are two testes 

 which vary greatly in size according to season. Two seminal 

 vesicles store the sperms. There are two vasa deferentia with 

 eversible papillae at the cloacal end. The Anatidae have a penis, 

 coiled when flaccid. The Cursores have a penis consisting of two 

 fibrous bodies with a fissure between. (Figures 204 and 205.) 



Female. — In the female the right ovary is degenerated^ with a 

 vestigial oviduct. The left ovary persists and has a long convoluted 

 oviduct with a papilla at the end. The oviduct secretes albumen 

 in the upper part, farther down the shell gland secretes the shell 

 membrane and finally the posterior part secretes the shell. The 



^ Great deposits of the feces and urine of certain birds are found in Peru. This 

 substance, rich in salts, is sold as the fertilizer, "guano." See page 368. 



