Can You Sex an Egg Before It Hatches?

Numerous folk methods floating around the internet claim to accurately determine the sex of an egg before or during incubation. But do they work? Can you really sex an egg before it hatches?

Technically the gender ratio of chicks at hatch should be 50/50. But for various reasons, some hatches skew one way or the other. If only we had a way to predict, and therefore influence, the outcome. Here are some prediction theories:

Egg Shape

An eggshell forms in the hen’s shell gland. Therefore the egg takes on the shape of the shell gland.

So each hen lays eggs of a characteristic shape. And by that shape you can usually identify which hen laid the egg.

Most eggs have a blunt end and a pointed end. But some eggs are nearly round, while others are more elongated.

If egg shape determines gender, some hens’ eggs would always hatch males. Others would always hatch females. That, in fact, is possible, just as some men always produce boys and others produce girls.

But with birds, the female, not the male, determines the offspring’s gender. So a particular hen could produce all (or mostly) males or all (or mostly) females. Some folks consider this proof that an egg’s shape determines a chick’s gender.

Aristotle (in The History of Animals, 350 BC) said: “Long and pointed eggs are female; those that are round, or more rounded at the narrow end, are male.” But wait! Pliny the Elder (in Natural History, AD 77) said: “Eggs of a rounder formation produce a hen chicken and the rest a cock.”

In 1931, University of Maryland poultry scientist Morley Jull published a paper on sex ratio in poultry in the Biological Bulletin. One section covered “Factors that Do Not Affect the Sex Ratio.” Included, with ample references, are egg length and shape.

Stubborn folks still won’t give up this notion. In 2002 a Japanese inventor filed a patent for a devise to measure egg shape. According to the application, “A method for determining the sex of a fertilized chicken egg, based on a shape of the egg, has been practiced for a long time in Kyushu or in Southeast Asia.” A further note says the method is both time consuming and inaccurate.

So the patent is for a device to measure egg shape, which is then statistically correlated with the gender of hatched chicks. Presumably, once enough data is collected, the correlation will be clear.

But a 2023 survey of egg sexing technologies, published in the Journal of Animal Science and Biotechnology, concludes there’s “Not enough scientific evidence that proves this technique works robustly nor an explanation of why there are sexual differences in the egg shape.”

Dowsing

Dowsing is a technique using a swinging pendulum to find something that is not readily visible. In this case, it is used to determine the gender outcome of a fertile egg.

Over the Internet you can waste your hard-earned money on pendulum sex indicators and magnetic hatching egg sexers. Some of them claim to come with a money-back guarantee. Old-timers used things like a needle and thread, a nail tied to a string, or a key hanging from a chain.

Simply dangle the pendulum over a fertile egg. If the pendulum swings back and forth, it’s a male. If it swings in a circle, it’s a female. Or maybe the other way around — back and forth for female, around for male — depending on who’s telling the story.

Despite this discrepancy, some people swear it works 100 percent of the time. Others insist it doesn’t work at all.

Maybe it depends on who’s handling the pendulum. After all, using rods to dowse for water works for some people and not for others.

Incubation Temperature

According to this theory, a hen that broods during winter will produce more pullets than a hen that broods during summer. The usual explanation is that a lower temperature causes fewer male embryos to survive. A high temperature causes fewer females to survive.

Likewise, eggs incubated at a temperature that is slightly lower than the manufacturer recommends will produce a greater percentage of pullets. Eggs incubated at a slightly higher temperature will produce a greater percentage of cockerels. The temperature deviation is a mere one-half degree Fahrenheit higher or lower than is optimum.

However, in 2013 the University of Georgia published a study that found no evidence that incubation mortality is sex-biased. Although incubation temperature does affect embryonic mortality, it does not affect the final sex ratio.

Emerging Technologies

If any of these folk methods were true, the poultry industry wouldn’t spend untold hours and dollars looking for easy, accurate ways to hatch only hens. Some of their proposed methods require injecting genes or dyes, or withdrawing fluids through the shell. Others call for cutting into the shell and examining or manipulating the contents.

Many non invasive methods are also under study, such as detecting the differing odors of volatile organic compounds emitted by developing embryos (otherwise known as the “Sniff Test”). Another non invasive method that’s catching on in the European Union is the use of hyperspectral measurement technology.

For this technique, a halogen light illuminates incubated eggs from below, while a camera takes images from above. The resulting images show each embryo’s down color, which reveals the embryo’s sex. Of course, this method works only for breeds and hybrids that can be color sexed at hatch.

Emerging industrial technologies are quite costly and therefore not available to the average backyard chicken owner. So here we are, left to wistfully fiddle with incubation temperatures, swinging needles, and egg shapes.

Portions of this blog are adapted from Hatching and Brooding Your Own Chicks by Gail Damerow.