418 PEARL— SEX RATIO IN DOMESTIC FOWL. 



perimental breeding operations with poultry at the Maine Agri- 

 cuhural Experiment Station during the breeding seasons of 1908 

 to 1915 inchisive. The 1916 matings are not included except for 

 the discussion of certain special problems because the original 

 record-taking on that year's birds is not completed at the time of 

 writing. During the period covered by the statistics the sex of 

 every chick which hatched was determined if it was physically 

 possible to make such determination. Failure to determine the sex 

 in individual cases resulted from one or another of the following 

 kinds of causes: (a) The loss of the bird from predaceous enemies, 

 thieves, or straying; (b) the bird's total destruction by fire; (c) the 

 loss of its identifying leg band, which rendered its assignment to 

 the proper mating impossible. In the case of birds which died be- 

 fore reaching an age where the development of secondary sex char- 

 acters made it possible to distinguish the sexes externally, dissec- 

 tion and examination of the gonads was resorted to for the deter- 

 mination. 



The number of cases of birds not sexed at all, for the reasons 

 above stated, was not proportionately large. I have elsewhere^ given 

 detailed figures on the point for one year. Other years presented 

 much the same sort of facts. The important feature is that these 

 irremediable losses, so far as all the evidence indicates, have been 

 random samples of the population in respect of sex. Further on 

 in the paper detailed evidence in support of this statement will be 

 presented. 



In the statistical treatment of the data the mating or family has 

 been made the unit, wherever such treatment is possible. While 

 not novel, this method of dealing with sex ratio statistics is unusual. 

 It has certain marked advantages, from a methodological viewpoint, 

 over the more usual procedure of considering a whole population 

 as the unit in studying the sex ratio. These advantages will be ap- 

 parent as we proceed. 



Throughout this paper the sex ratio is presented as the percentage 

 of the males in the total of the group or population. Or, in other 

 words, we express the sex ratio as 



:^ Pearl. R., Amcr. Nat.. Vol. XLV., pp. 107-117. 1911. 



