Is the sex ratio of Japanese quail offspring equal?

Offspring sex ratios in avian species are of significant scientific interest, with implications for evolutionary biology and poultry production. This study investigated sex ratios in Japanese quail (Coturnix japonica), a valuable model for other poultry species due to its rapid generation interval. The study examined the impact of selection over generations, age at first egg (AFE), and body weight at AFE (BW_AFE) on offspring sex ratios. The dataset included 4,282 Japanese quail records from 968 dams over eight generations, comprising two lines: one selected for high growth rate during 1-21 days of age and an unselected control line. Offspring sex ratio data were categorized based on dam characteristics: AFE (early: <48 days, medium: 48-52 days, late: >52 days) and BW_AFE (low: <249 g, medium: 249-268 g, heavy: >268 g). These categories represent below average, average, and above average values for each parameter, respectively. Analyses were done on pedigree and hatching records from two lines of selected and control quails. The chi square and logistic regression analyses exhibited insignificant associations between the examined predictor variables (generation, line, AFE, and BW_AFE) and the sex ratio outcome in Japanese quail. Therefore, it can be concluded that the proportion of male and female offspring quail in the flock is statistically equal. However, regarding the BW_AFE categories the residual analyses revealed a potential tendency toward a male-biased sex ratio within the medium category also, they suggest potential tendencies toward male-biased (eighth generation) and female-biased (sixth generation) sex ratios that warrant further investigation.