Birds start off with two embryological ovaries but in general only one - the left develops to maturity and the right one remains rudimentary. If however the left ovary is destroyed by disease or deliberate experimental technique then the right ovary will develop, not into a mature Ovary, but into a functional testis! In this way a complete sex reversal can occur in some species of domestic fowl. An erstwhile female may acquire cock plumage and tread and fertilise hens. There are instances on record where an individual started off by laying eggs and finished up fertilising females. Sex reversal very rarely occurs in the opposite direction.