After six months following circuses around Britain, Birkett joined the Scott circus in Sweden, where the form has a massive popular following and the ringmaster is treated like a film star. For just one night, Birkett wore a gold spangly dress and led the elephants into the ring for their nightly performance. Her job was to reward them with pieces of bread every time they performed correctly.
"It was magical. I was breathless. I cannot describe how wonderful it was to be surrounded by these enormous animals who are so powerful but so gentle," she says. "I have always wanted to stand in the middle of the ring and be applauded by a crowd who are genuinely transported by what they have seen.
"The elephants caress you with their trunks and afterwards you smell of elephant sweat, which is really musky, and your clothes are sticky from their dribble, but you don't mind because the whole thing was so wondrous."
Watching the animals in rehearsal, Birkett became convinced that they loved performing. "Elephants are really very soppy and gentle, but they have moods just like everyone else. If they don't want to do it then they won't, and no amount of persuasion is going to change that. It's their call.
"One elephant, called Kitty, went off in the middle of rehearsal and starting looking round the edge of the ring for popcorn left from the night before. Nobody minded because they knew she'd come back when she was ready, and she did.
"The trainers love their animals and have a lot of respect for them. The elephant tent is just two or three yards away from the caravan where the elephant trainer lives. He is always there. When I first stayed the night with a circus troupe I didn't sleep at all; the animals are so close and you can hear them every time they move in the night. This closeness breeds an incredible intimacy between man and beast. You smell animals all the time - you never leave it."
On her first day at the Scandinavian Circus Scott, which is sponsored by IBM (a marketing team that alone illustrates how massive the Scandinavian circus is), she worked with Marco the elephant trainer.
"Everyone has this image that elephants are trained with sticks and whips, but he just opened the boot of his car and it was full of stale bread. I'd brought a packet of jelly beans but he said that was bad for their teeth and they preferred bread. It's all part of the real concerns they have for the animals." The Chipperfield case which caused such a stir in this country is, says Birkett, an anomaly.
"I spent six months with circuses and I never saw any cruelty at all."
She is convinced that the cruelty filmed by inspectors at the Chipperfield circus was uncharacteristic of circuses.
"I think the court made the right decision and I don't defend her, but I don't think that because there are isolated incidents the whole industry should be condemned. There is no doubt that the animal activists have won the propaganda war and circuses are under siege in this country. They are being harassed and hounded and treated like outlaws - but they are pursuing a perfectly legal profession and the activists refuse to have a reasoned debate about it."
Birkett has certainly spent more time with the circus than most opponents who hold equally trenchant views, but the animal rights campaigners question how much reality she saw.
Diane Westwood, of the Captive Animals Protection Society, says that animals perform when their spirits have been broken and they carry out the act by rote.
"The saddest part of the circus is the training, and that is the cruellest part. We have spoken to lots of ex-animal trainers and they have told us about it. It's likely that Dea didn't see any cruelty, but that doesn't mean there wasn't any behind the scenes. She wasn't there all the time.
"People are beginning to understand that animal circuses are wrong. After all, in the 1800s people regularly gathered in their local market square to watch criminals being hanged, and they thought that was entertainment."
Jan Creamer, director of the pressure group Animal Defenders, which obtained video evidence that led to Chipperfield's conviction, says that no circus animal ever has enough space to live in. "Dea is seeing what she wants to see. The reality is that all the animals are kept in small cages and spend a lot of time travelling around. Even if the trainers do love their animals - and some of them do - they don't have the facilities to look after them properly." But Birkett describes a coherent community centred entirely on the circus. "These people live in caravans for at least nine months of the year. If you ask them where is home, they say `the circus'. If you ask them what is their nationality, they say `I'm from the circus'. There are over 20 nationalities in one circus. If the Moroccan tumbler marries the French trapeze artist, their children are from the circus. Their notion of home is a community of people."
Sitting on the Tube platform in south London, she continues to feel an acute nostalgia. "When the circus moves on, every trace of it disappears. Even the holes from the tent pegs are filled in and it's as if it was never there. It's like a dream and I long to return."
`Circus Days Circus Nights' is on Channel 4 on Tuesday at 11.35pmReuse content