BIRDS' EGGS 281 



§ II. Fertility and Fecundity 



Comparatively little is known in regard to the fertility 

 of birds and the conditions which cause it to vary. Most 

 of what is known is due to the investigations of fowls by 

 Professor Raymond Pearl and his fellow-workers. The 

 maximum total number of chicks physiologically possible to 

 a mated pair is one thing, and the actual number of viable 

 chicks is another, the relation of the actual to the possible 

 is called by Pearl (19 17) the reproductive or fertility index. 

 It diminishes after the first breeding season. 



In mammals it seems that fertility, starting at a low point 

 at the beginning of the sexual life, rises to a maximum at a 

 certain age, and then declines with further increase in age, 

 until total sterility results. In the fowls Pearl (1917, A) 

 worked with, the rule was very different. There is in both 

 sexes a steady and progressive decline in fertility after the 

 first breeding season. It is more rapid in the male than in 

 the female. " There is a significant drop in reproductive 

 ability as we pass from a combined age of two years for the 

 mated birds to three years. In passing from three years to 

 four there is no significant change in reproductive ability ; 

 on passing from a combined age of four years to that of five 

 years, there is a large drop in the net reproductive ability 

 of the mating." 



The female's capacity for producing oflPspring is called 

 fecundity ; the actual number produced indicates the 

 fertility. A hen may fail to lay anything like the number of 

 eggs which she is capable of producing. She may not be as 

 fertile as she is fecund. A reliable estimate of this fecundity 

 can be obtained by counting the number of readily visible 

 immature ova (oocytes) on the surface of the ovary (Pearl 

 and Schoppe, 1921). For when the ovary is removed it is 

 seen to be like a bunch of grapes. It is made up of many 

 oocytes held together by connective tissue and bound to a 

 stalk which keeps them in position in the body. There are 

 many immature ova which are quite hidden, many that will 



