Spontaneous sex changes can happen in chickens with female hens developing the characteristics of male roosters.

Chickens can undergo this natural sex change when their only functional ovary becomes damaged or diseased. In these cases, their other dormant gonad may begin producing male hormones, which trigger male behaviors and features. The hen may develop male secondary sex characteristics like growing larger wattles, changing the color of their plumage, and starting to crow. 

Despite these changes, the chicken remains genetically female and cannot father chicks—only their appearance and behavior change. 

This is a rare but natural phenomenon.

Learn more:

 >> https://poultry.extension.org/articles/poultry-anatomy/avian-reproductive-female/sex-reversal-in-chickens-kept-in-small-and-backyard-flocks/

>> https://www.livescience.com/13514-sex-change-chicken-gertie-hen-bertie-cockerel.html