Incubation Temperature Does NOT Determine Chick Sex (But It Does in Reptiles)
What the research actually shows—and why this myth continues to circulate
A persistent belief in the poultry world is that incubation temperature can influence whether chicks hatch as male or female. You’ll hear claims that higher temperatures produce more hens or lower temperatures produce more roosters.
This idea is incorrect.
Scientific evidence consistently shows that incubation temperature does not determine sex in chickens, quail, or any other bird species.
Understanding why requires separating two very different biological systems: genetic sex determination in birds and temperature-dependent sex determination in reptiles.
How Sex Is Determined in Poultry
In birds, sex is established at the moment of fertilization.
Males carry two Z chromosomes (ZZ)
Females carry one Z and one W chromosome (ZW)
This system is fundamentally different from mammals and is controlled by genetic pathways, including the gene DMRT1, which plays a central role in male development (Smith & Sinclair, 2004).
By the time an egg is laid, the embryo’s sex is already fixed. Incubation conditions cannot alter that outcome.
What Research Demonstrates
Controlled studies have directly tested whether incubation temperature can shift sex ratios in poultry.
Collins et al. (2013) examined broiler eggs incubated at varying temperatures and found that while temperature influenced embryo development and hatchability, it had no effect on the proportion of males to females.
Their conclusion was clear: incubation temperature does not influence sex ratio in chickens.
This finding is consistent across avian research. Göth et al. (2004) reported no evidence that temperature affects sex ratios in birds, and Koláčková et al. (2019) confirmed that temperature-dependent sex determination is absent in avian species.
Across the literature, the conclusion is consistent: bird sex is genetically determined and not environmentally controlled during incubation.
Why the Myth Persists
The confusion comes from a real but misunderstood phenomenon.
Incubation temperature does influence embryo survival. When temperatures drift outside optimal ranges, embryos become stressed, and mortality increases.
In some cases, one sex may be more sensitive to temperature stress than the other. This can result in a hatch that appears skewed toward males or females.
However, this is not because temperature changed the sex of the embryos. It is because one sex experienced higher mortality during development.
This is known as sex-biased embryonic mortality, and it is fundamentally different from sex determination.
The sex ratio at fertilization remains unchanged. What changes is which embryos survive to hatch.
Where Temperature Does Determine Sex: Reptiles
The belief that temperature influences sex originates from reptile biology, where it is entirely valid.
Many reptiles use a system known as temperature-dependent sex determination (TSD).
In these species, incubation temperature directly influences hormonal pathways during development, which determines whether the embryo develops as male or female.
For example, in many turtles, higher incubation temperatures produce females, while lower temperatures produce males. In alligators, intermediate temperatures tend to produce males, while both higher and lower extremes produce females.
This process is mediated by enzymes such as aromatase, which regulates the conversion of androgens to estrogens during embryonic development.
Birds do not use this system. Their sex is not influenced by incubation temperature at any stage.
Why This Matters for Poultry Keepers
Attempting to manipulate sex ratios through incubation temperature leads to poor outcomes.
Elevated or reduced temperatures outside optimal ranges can decrease hatchability, increase developmental abnormalities, and weaken chick viability.
Rather than influencing sex, improper temperature control introduces unnecessary stress into the incubation process.
If a breeder observes repeated skewed sex ratios, the cause is more likely related to factors such as breeder flock condition, nutrition, egg handling, or storage practices—not incubation temperature.
The Bryant’s Roost Approach
Consistent hatch results come from controlled systems, not folklore.
Temperature should be managed to support embryo development, proper moisture loss, and a stable hatch window. It should not be treated as a tool for influencing sex ratios.
Understanding the difference between genetic determination and environmental stress is essential for improving hatch outcomes and maintaining flock quality.
Frequently Asked Questions
Does higher incubation temperature produce more hens?
No. Scientific studies show that incubation temperature does not affect sex ratio in poultry.
Can you control chick sex through incubation?
No. Sex is determined at fertilization through the ZZ/ZW chromosomal system.
Why do some hatches have more males or females?
This is typically due to differential embryo survival, not a change in sex determination.
Do any animals use temperature to determine sex?
Yes. Many reptiles, including turtles and crocodilians, use temperature-dependent sex determination.
What happens if incubation temperature is too high?
High temperatures can reduce hatchability, increase embryo mortality, and lead to developmental defects.
Key Takeaway
In poultry, sex is determined genetically at fertilization. Incubation temperature affects survival and development, but it does not determine whether a chick hatches male or female.
References
Collins, L. M., et al. (2013). Incubation temperature does not influence sex ratio in broiler chickens. Poultry Science.
Göth, A., Booth, D. T., & Evans, A. (2004). Temperature-dependent sex ratio in a bird. Proceedings of the Royal Society B.
Koláčková, M., et al. (2019). Evolution of sex determination systems in amniotes. Developmental Biology.
Smith, C. A., & Sinclair, A. H. (2004). Sex determination in the chicken embryo. Journal of Experimental Zoology.
Wilson, H. R. (1991). Interrelationships of egg size, chick size, posthatching growth and hatchability. World’s Poultry Science Journal.
Deeming, D. C. (1995). Factors affecting hatchability during incubation. Poultry and Avian Biology Reviews.