Does the temperature in the incubator determine the sex of a chicken? Last spring we incubated some eggs and ended up with a bunch of roosters. We turned the temperature down to about 99°F and ended up with mostly hens and a few roosters.
The effect of temperature on the sex of the offspring is related to reptile eggs (such as alligators) and not for birds.
The sex of a bird is determined genetically and is set before the egg is even laid. Unlike mammals, however, it is the female bird that determines the sex of the offspring and not the male. In mammals the male is heterozygous (XY) and the female homozygous (XX) so that the male is the only parent with the Y gene which will result in a male offspring. In birds the female is heterozygous (ZW) and the male is homozygous (ZZ) so that the female determines the sex of the offspring.
While the sex of the chick is determined before the egg is laid, research has shown that male and female embryos may differ in their sensitivity to suboptimal conditions during embryo development. As a result, the ratio of males and females that actually hatch can vary depending on incubation conditions. This is because the ratio of males and females that do not hatch is affected not because the sex of the embryos was changed by the temperature (as happens with some reptiles).
Genetics of sex determination in mammals versus birds
MAMMALS
(e.g., humans, cows, etc.)
Male = XY
Female = XX
Males determine the sex of the offspring
BIRDS
(Including poultry)
Male = ZZ
Female = ZW
Females determine the sex of the offspring