Niffenegger devised the book, illustrated with her own etchings and aquatints, both as a ballet scenario and independently from it. "In ballet you have the emotional impact of real people doing extraordinary things right there in front of you, and in the music as well," she says. "The vocabulary of dance is so different from anything you can do in a book - they're almost opposites - and that's why it's been interesting to work with Wayne from the beginning, knowing that the story's going to be a ballet, rather than adapting something I had already written."
Up to a point, she says, she was inspired by those other famed bird-women in ballets Swan Lake and The Firebird. "In the most general way I was thinking, 'ballet is full of birds'. But usually the birds are much more glam. Ravens, on the one hand, are incredibly elegant creatures, yet the way they move is not elegant and their voices are very rough.
On the other hand, they're amazingly intelligent. I liked them because, although the behaviour in the story is anthropomorphised, it's not such a big leap from that to imagine ravens having their own societies and behaving in the ways they do in this story."
At first Niffenegger expected she would need to create a danceable narrative: "I felt slightly constricted by that idea, but I spoke to Wayne and he told me not to worry; he said he would take care of making it danceable. That was very freeing.
He was good to let the book be what it is. We both knew from the beginning that the book would be one thing and the dance would take its own shape.
"The great thing about everything I've seen Wayne do is that it's often surprising.
Sometimes that's because you're thinking, 'My God, the body can do that?' And sometimes it's just a really interesting or elegant combination of movements."
She was amazed to see the final pas de deux in rehearsal. "Something that was just one image in the book - the Raven Girl flying with the Raven Prince - has turned into about 10 minutes of dancing. The level of emotion is greatly expanded."
As for the idea of a human being sensing a wild bird within, the metaphor offers something for everyone, not least for Niffenegger herself. "For me it's about a girl who's a misfit and feels something is not quite right. As a kid, I was the weirdo. My friends didn't go to my school, so I'd sit there all day as the odd-one-out."
McGregor says that Niffenegger's aquatints were a particularly powerful inspiration. "Audrey's a phenomenal visual artist," he says. "There is something peculiar about the way she orientates figures on a page and about the way she gives her tonal palette an emotional and narrative weight. That's something you can really do well on a stage. I love her images that sometimes look half-finished, with a very small amount of text - you have to construct a meaning between the two."
It is one matter for Niffenegger to imagine and illustrate Raven Girl's surgical transformation, but quite another to stage it, Ballerina Sarah Lamb, who is creating the role, will be donning a contraption that gives her a 3.5m wingspan.
The designs are by Vicki Mortimer who, Niffenegger says, "is not just taking ideas from the illustrations in the book, but also taking up images I have only described in words, and realising those on stage.
"I feel so lucky," she adds, "because Wayne has assembled a group of people who blow my mind every day in every way."
• Raven Girl (Jonathan Cape $45) is out now.
- Independent
