http://www.xanga.com/home.aspx?user=chocsauce&nextdate=7%2F26%2F2004+0%3A51%3A35.670
This a pretty strange website, but has the following information, which sounds reasonable enough:
How does sex reversal occur?
Most cases of spontaneous sex reversal are the result of a disease condition which has resulted in damage to the left ovary. Typically, female chickens only have one functional ovary, the left one. The right ovary and oviduct are present in the embryonic stages of all birds, but typically do not develop in chickens.
In general, spontaneous sex reversal has been described as the result of pathological conditions (e.g., ovarian cyst or tumor, diseased adrenal glands) which cause the left ovary to regress. Residual tissue in the right ovary proliferates in the absence of a functional left ovary. This regenerated right gonad is known as an ovotestis and may contain tissue characterisitics of the ovary, the testes, or both. There are reports of these ovotestes producing semen capable of fathering offspring. Most, however, will never lay an egg or sire offspring.
The “ovotestes” are steroidogenically functional and secret androgens, as well as estrogen. As a result, the bird develops male secondary characteristics. So while the bird is genotypcially female, it will be phenotypically male.