Daily Mail's take - it's one of the more informative:
The confident strut, distinctive wattle and comb and ear-splitting crowing all suggest this bird is a cockerel.Now the Beeb's :
There is just one problem – a few months earlier he was a she.
The extraordinary transformation has happened since November, when the speckled hen, called Gertie, stopped laying eggs and began developing male characteristics.
According to Victoria Roberts, of the Poultry Club of Great Britain, the spontaneous sex change is ‘a one in 10,000 event caused by changes in the bird’s hormones’.
Damage to an ovary can cause the bird’s testosterone levels to soar, turning the remaining ovary into a testicle.
Delia Richter, a vet at Cromwell Vets in Huntingdon, said damage to the hen's single ovary or a growth upon it could cause it to exhibit male characteristics.
Miss Richter said: 'It would still be a hen but the ovary on the left side degenerates and the right side begins to release testosterone.
'It's possible that's what happened in this case.'
Cambridge veterinary surgeon, Marion Ford, told the BBC that an apparent sex change in hens was not uncommon.More details from the Daily Express:
She said that it could be caused by mycotoxins or fungi that can develop when animal feed is stored, and these have the same effect as synthetic hormones.
"An increase in testosterone will result in a hen growing an extended comb, exaggerated wattles, and cockerel-like behaviour including strutting and crowing," explained Mrs Ford.
Mrs Howard said: "I'm not really sure whether Gertie has actually changed sex, but to all intents and purposes she's now a cockerel."
It is not known whether the effects of the mycotoxins are reversible.
Victoria Roberts, of the Poultry Club of Great Britain, said it was a “very rare” hormone change.That sounds a bit more likely, but comments by poultry breeders suggest 1 in 10,000 is too low, rate is much higher.
She said: “Only one ovary functions in a chicken. If that gets damaged the other kicks into life but causes a jump in testosterone which sparks changes. They cannot reproduce.”
Going back to 2006... and the Grauniad:
But have they undergone an actual sexual transformation, or are they just hens in cocks' clothing? "The crude mechanics of it are that every hen bird has two ovaries, one is big and functional and the other is benign, and the benign one develops testosterone," explains Taylor. Accordingly, the hen acquires the physical characteristics of a cockerel. The British Poultry Council says the same change can also occur if the active ovary is damaged and the hen begins to rely on her reserve ovary. "This can either be due to an imbalance in oestrogen or something that has promoted testosterone. But, although a hen may take on the secondary sexual characteristics of a cockerel, it will not develop the primary sexual organs." A cock without a cock, so to speak.Hur Hur. What captivating wit.
From J.Elliot of "Ask a Scientist" Office of Science, US Department of Energy :
I don't believe it is possible for any birds (or other higher vertebrates) to change gender. Many weaver finches are gregarious and polygonous so there may be some confusion regarding behavior. Orange weaver finches are common cage birds and I think if any gender change had been observed it would be well documented, but I am not familiar with cage birds.So who to believe - the scientist or the tabloids? Neither - I believe the testimony of the bird-breeders and the veterinarians, the ones who actually make observations of the data, and the ones who have actually studied the area. And of course, the birds themselves.