The Sex Ratio in Wild Birds

Abstract

Strongly unequal sex ratios occur not uncommonly in birds. Such sex ratios are either high or low, favoring the [male] or the [female] sex. The evidence points to the existence of this unbalanced condition already at the time of fertilization (primary sex ratio), although genetic and environmental factors may modify it during prenatal and postnatal life. The more important known cases of unbalanced sex ratios in birds are listed and the evidence is analyzed. In most cases they have been found to be correlated with peculiarities in the life history of the birds, such as polygyny, polyandry or promiscuity. To account for unbalanced sex ratios in birds, the occurrence of sex modifying factors is postulated which either affect the oocytes prior to ovulation or which operate during fertiliza-tion.