Q: How can I incubate eggs that will hatch female chicks only?
A: The question of temperature-dependent sex determination (TSD) in chickens has been an important subject for scientific study over the last several decades. For most backyard chicken enthusiasts, females are preferred for egg laying and unwanted roosters often have to be rehomed. Wouldn't it be great if you could incubate and hatch all females, or at least hatch a much higher ratio of females than males? Sure!
But as we shall see--don't get your hopes up just yet...
An Australian study published in 1960 (Australian Journal of Agricultural Research 11(4) 664 - 672) found that eggs stored at 40 degrees Fahrenheit (rather than the recommended 60 degrees) for at least 24 hours and up to 7 days before incubation hatched out 54.6% more females than males. It was postulated that the male embryos were preferentially killed off by colder storage temperatures, while the female embryos handle the chilling better. At the time of this writing, we have not found a study that replicated this result.
Another study (Ferguson, M. W. J. 1994 Temperature dependent sex determination in reptiles and manipulation of poultry sex by incubation temperature. In Proc. 9th European Poultry Conf., Glasgow, pp. 380–382) purportedly demonstrated sex reversal in chicks in ovum by pulsing eggs with higher or lower temperatures, but the results were never replicated.
A much more recent study published in 2013 (Poultry Science, Volume 92, Issue 12, 1 December 2013, Pages 3096–3102, https://doi.org/10.3382/ps.2013-03378) found no evidence of temperature-dependent sex determination or sex-biased embryo mortality in the chicken.
At this time, unfortunately, there is no replicated empirical evidence of temperature-dependent sex determination (TSD) or sex reversal in chicken embryos, but poultry scientists are always working on this question, so perhaps an answer is forthcoming?
A potentially more helpful solution would involve the ability to tell the sex of an embryo in ovum before placing it in the incubator. Thankfully, that technology is being developed, but unfortunately is not available to most hobbyists and hatcheries at this time, as you can read in the "related questions" below. But hopefully, someday the technology will be more readily available!