DESCRIPTION OF PROBLEM In

a comparison between male and female broiler chicks, the mortality rate of males was found to be significantly higher than that of fe-males, beginning from the second week of age until birds were marketed at 7 or 8 wk of age [1]. this difference in mortality rates of chicks may result from a less efficient immune response in males [1]. During incubation, many differences in embryo development exist between males and females. For instance, Henry and Burke [2] found that male embryos were significantly heavier than female embryos at 16 d of incuba-tion, and the cross-sectional area of myofibers of 20-d-old female embryos was greater than that of males. Furthermore, Glahn et al. [3] found that sex-related differences in heart rates existed in embryos. Female embryos exhibited higher mean heart rates than males from d 15 to 19 of incubation. Because of these sex-related differ-ences in chicken embryo development, our aim © 2012 Poultry Science association, Inc. Sex-biased mortality analysis in chick embryos during the entire period of incubatio