FERTILITY IN RHODE ISLAND REDS . 7 



yearlings. Yearling males gave the highest fertility from yeai ling hens in the 

 later hatches, but the number of tests is too small for significance in the earlier 

 hatches. T\vo-> ear-old males gave the highest fertility when mated to females 

 of their own age. The very old males appeared to give highest fertility when 

 mated to yearlings. Females two years old or over gave the highest fertility record 

 when mated to cockerels. Since the data indicate that fertility declines consistently 

 with age in males and to a lesser degree in females, a mating between young 

 birds and old birds might raise the fertility. When, however, the physical condi- 

 tion of the birds is superior and environmental conditions are right, the mating 

 of aged birds will generally give satisfactory fertility. 



3. Relation Between Sex and Fertility 



It is desirable to know whether a difference exists between the sexes with 

 respect to fertility. Crew (1926) pointed out selective fertilization on the basis 

 of sperm competition when two different males were used. Dunn (1927) pre- 

 sented data indicating selective fertilization on the basis of relationship of the 

 birds mated. He secured higher fertility from related matings within a breed 

 than from breed crosses. Warren and Kilpatrick (1929) found some evidence of 

 selective matings and very conclusive evidence of sperm competition. 



Selective mating occurs rather frequently in most poultry flocks. In almost 

 any series of mating pens females will appear that never lay fertile eggs. Such 

 infertility may be caused by selective mating either on the part of the female 

 or on the part of the male or possibly by incompatibility between sperm and ova. 

 The data in Table 2 show that increased age has a greater effect upon the fertility 

 of males than upon the fertility of females. 



The fertility records of the individual males were tabulated by years. The 

 fertility of individual males represents an average of the fertility of. all of his 

 mates. These records did not show a normal frequency distribution at any 

 time, but for the most part approached a bimodal distribution with a rather con- 

 spicuous separation at the 60 percent fertility class. This bimodal distribution 

 suggested the possibility of two genetic phenotypes for fertility. The minor 

 irregularities of the curves pointed to other modifiers, some of which may have 

 been genetic. The fact should be noted in this connection that males and females 

 have never been selected directly for high fertility. There has been indirect 

 selection in that birds have been chosen from large families and older breeding 

 males, and females were not used unless they had shown the ability to produce 

 abundant offspring. 



The mean percentage of fertile matings for the males used throughout the 

 fourteen years is recorded in Table 3 to show how fertility behaved from the 

 standpoint of the males. The percentage of males with a fertility record of 61 

 percent or higher is also recorded. These data do not point to any significant 

 change in the appearance of fertile matings in the first ten-year period. In the 

 earlier hatching period of the last four years the males had a somewhat lower 

 rating, but within this period there was relatively little change in the mean per- 

 centage of successful matings. 



The mean percentage of males with a fertility record exceeding 60 percent 

 fluctuated widely throughout the period but showed no specific change in the 

 ten-year period. In the early-hatched group of the last four years, the percentage 

 of superior males fell to a low level and remained there. In general, the percentage 

 of successful matings and the percentage of superior males changed little during 

 the period studied. 



