Chick sexing is the method of distinguishing the sex of chicken and other hatchlings, usually by a trained person called a chick sexer or chicken sexer. Chicken sexing is practiced mostly by large commercial hatcheries to separate female chicks or "pullets" (destined to lay eggs for commercial sale) from the males or "cockerels" (most of which are killed within days of hatching because they are irrelevant to egg production). The females and a limited number of males kept for meat production are then put on different feeding programs appropriate for their commercial roles.
Different segments of the poultry industry sex chickens for various reasons. In farms that produce eggs, males are unwanted; for meat production, separate male and female lines for breeding are maintained to produce the hybrid birds that are sold for the table, and chicks of the wrong sex in either line are unwanted. Chicks of an unwanted sex are killed almost immediately to reduce costs to the breeder.
Methods of chick sexing
Vent sexing, also known simply as venting, involves squeezing the feces out of the chick, which opens up the chick's anal vent (called a cloaca) slightly, allowing the chicken sexer to see if the chick has a small "bump", which would indicate that the chick is a male. Some females also have bumps, though they are rarely as large as those of male chicks.
The eminence or genital organ is found midway on the lower rim of the vent, and looks like a very small pimple. Most males have a relatively prominent eminence, most females have none. However, a small proportion of both males and female have relatively small eminences. Sexing these chickens can be quite difficult, but with regular practice, the sexer will eventually learn to identify the differences.
When learning to sex chickens, it is best to assume that chickens with small eminences are female. The male eminence is solid and will not disappear upon gentle rubbing with one's thumb.
A paper about vent sexing was published in Japan in 1933 by Professors Masui and Hashimoto, which was soon translated into English under the title sexing baby chicks. After their discovery, interested poultry breeders hired those who had been trained in Masui and Hashimoto's technique, or sent representatives to Japan to learn it. 
(See also Delayed feathering in chickens)
In 1969, after three years of genetic research, Tegels Poultry Breeding Company developed broiler chickens that could be feather-sexed. The result was a strain that would produce slow-feathering males and fast-feathering females.
In the slow-feathering males, the coverts are either the same length or longer than the primary wing feathers. In the fast-feathering females, the primary wing feathers are longer than the coverts. This is caused by a gene located on the sex chromosome where slow feathering is dominant to rapid feathering and controls the rate of wing and tail feathering in the chicken. The dominant slow-feathering characteristic is passed from mothers to their sons and the rapid feathering characteristic from the fathers to their daughters. Fathers also pass the rapid-feathering gene to their sons, but this is not expressed because it is recessive.
Small poultry farmers whose operations are not of sufficient size to warrant hiring a chicken sexer must wait until the hatchlings are four to six weeks old before learning the sexes of their chickens. At that time, their secondary sex characteristics begin to appear, making it possible for anyone with a minimal amount of training to sex a chicken.
Instrument or machine sexing of chickens has almost disappeared, because the instruments are no longer available and spare parts cannot be obtained. The Keeler Optical (English) or Chicktester (Japanese) machine features a blunt-ended telescopic tube, containing a light. The sexer inserts the tube into the evacuated cloaca and with the help of the light can identify either testis or ovaries. Successful development of this technique depends on the capability of the students and their level of experience.
- Kemsley, Max (3 June 2010). "Chicken sexing". Department of Agriculture, Fisheries and Forestry (Queensland). Retrieved 13 January 2013.
- Hatchery Horrors: The Egg Industry's tiniest victims. Mercy for Animals.
- New Investigation Reveals Horrific Cruelty at 'Humane' Chicken Hatchery. Mercy for Animals. Retrieved 17 October 2013.
- Robinson Bosk, Beth. "Sexing Day-Old Chicks: How to Identify Pullets and Cockerels". motherearthnews.com. Retrieved August 30, 2015.
- Lloyd, Prof. E.A. "Sexing Baby Chicks". AG Annex. Retrieved August 30, 2015.
- Poultry: Sexing of day-old chicks as a sport
- How to tell the sex of Chicks - including general signs, breed specific tips and old wives tales.
- The art of chicken sexing - full article (PDF)