Chickens can sometimes undergo natural sex changes. Normally, female chickens have just one functional ovary, on their left side. Although two sex organs are present during the embryonic stages of all birds, once a chicken's female hormones come into effect, it typically develops only the left ovary. The right gonad, which has yet to be defined as an ovary, testes, or both (called an ovotestis), typically remains dormant. Certain medical conditions can cause a chicken's left ovary to regress. In the absence of a functional left ovary, the dormant right sex organ may begin to grow; if the activated right gonad is an ovotestis or testes, it will begin secreting androgens. The hen does not completely change into a rooster, however. This transition is limited to making the bird phenotypically male.[5][6] The condition could also be caused by mycotoxins that can develop when animal feed is stored, and these have the same effect as synthetic hormones.[7] In about 10 percent of cases, if eggs fertilized with male chromosomes are cooled by a few degrees for three days after laying, the relative activity of the sex hormones will favour development of female characteristics. The sex chromosomes work by coding for enzymes that affect the bird's development in the egg and during its life. This cooling will produce a chicken with a fully functioning and reproductively fertile female body-type, even though the chicken is genetically male.

https://en.wikipedia.org/wiki/Sex_change#Natural_sex_change