The left, white, side of this bird is male. The right, brown, side is female.
By Huw Williams
BBC Scotland reporter
Researchers say they've solved the mystery of why some chickens hatch out half-male and half-female.
About one in every 10,000 chickens is gynandromorphous, to use the technical term.
Half-and-half chickens give a unique insight into how birds develop
In medieval times, they might have been burned at the stake, as witches' familiars.
But now these chickens are shedding important new light on how birds, and perhaps reptiles, develop.
It used to be thought that hormones instructed cells to develop in male or female-specific ways.
That's what happens in mammals, including humans, and it leads to secondary sexual characteristics like facial hair for men or breasts for women.
But scientists at the Roslin Institute and the University of Edinburgh say they have discovered that bird cells don't need to be programmed by hormones.
Instead they are inherently male or female, and remain so even if they end up mixed together in the same chicken.
It means a half-and-half chicken will have totally different plumage, body shape, and muscle structure on the two halves of its body.
It even affects the wattles on the bird's head, and the spurs on its legs. They will be larger on the cockerel half, and smaller on the hen half, of the same bird.
Dr Michael Clinton of the Roslin Institute led the research, which has just been published in the scientific journal Nature.
He said the findings were a surprise.
Dr Clinton explained: "We looked at these birds initially expecting them not to be half-male and half-female. We thought there'd be a mutation on one side of the body.
Dr Michael Clinton of the Roslin Institute led the research
"But we found that they were half-male and half-female and that's what actually showed us that the system was different in birds and mammals."
And researchers tested their theory with delicate and demanding experiments.
"If you put female cells into a male body they'll develop into the normal tissues, but they'll behave as female cells," Dr Clinton said.
The hope is that these findings might have immediate practical uses for the poultry industry.
Dr Clinton said: "If we can understand what the differences between the male and female identities are, then we can imagine making female birds with the same growth characteristics as males. That would increase productivity, and food security."
But if there are vestiges of the same mechanism in mammals, inherited from our reptilian evolutionary ancestors, then the research could help to answer long-standing mysteries of human health.
Like, for example, why women live longer than men, or why men are more at risk of heart attacks.
"But that will require much more investigation," Dr Clinton insisted.