A few months ago, in a cafe in St. Louis, I met a man named Jim Eggers, who uses an assistance parrot, Sadie, to help control his psychotic tendencies. Eggers looks like a man who has been fighting his whole life. He is muscular, with a buzz cut, several knocked-out teeth and many scars, including one that runs ear-to-chin from surgery to repair a broken jaw. Eggers avoids eye contact in public — he walks fast down streets and through stores staring at the ground, jaw clenched. “I have bipolar disorder with psychotic tendencies,” he told me as he sucked down a green-apple smoothie. “Homicidal feelings too.”

Eggers’s condition has landed him in court several times: a disturbing-the-peace charge for pouring scalding coffee onto a man under his apartment window who annoyed him; one-year probation for threatening to kill the archbishop of St. Louis because of news reports about church money and molestations by priests in other cities (which the archbishop had nothing to do with). In describing his condition, Eggers says it’s like when the Incredible Hulk changes from man to monster. His vision blurs, his body tingles and he can barely hear. According to his friend Larry Gower, who often serves as a public liaison for him, in those moments, Eggers gets extremely loud. They both agree that Sadie is one of the few things keeping Eggers from snapping.

Sadie rides around town on Eggers’s back in a bright purple backpack specially designed to hold her cage. When he gets upset, she talks him down, saying: “It’s O.K., Jim. Calm down, Jim. You’re all right, Jim. I’m here, Jim.” She somehow senses when he is getting agitated before he even knows it’s happening. “I still go off on people sometimes, but she makes sure it never escalates into a big problem,” he told me, grinning bashfully at Sadie. “Now when people make me mad I just give them the bird,” he said, pulling up his sleeve and flexing his biceps, which is covered with a large tattoo of Sadie.

Soon after what he calls “the Archbishop Incident,” Eggers got Sadie from a friend who owned a pet store. She’d been neglected by a previous owner and had torn out all her feathers, so Eggers nursed her back to health. He didn’t initially train her as a service animal, he says; she did that herself. When Eggers had episodes at home, he’d pace, holding his head and yelling: “It’s O.K., Jim! You’re all right, Jim! Calm down, Jim!” One day, Sadie started doing it, too. He soon realized that she calmed him better than he calmed himself. So he started rewarding her each time it happened. And he has had only one incident since: he dented a woman’s car with his fist on a day when he’d left Sadie at home.

Eggers didn’t think to use any special language to describe Sadie until he tried to take her on a bus and the driver said that only “service animals” were allowed. Eggers went home and looked up “service animal” online. “That’s when it all fell into place,” he told me. He learned that psychiatric service animals help their owners cope with things like medication side effects. Eggers takes heavy doses of antipsychotics that leave him in a fog most of the day. So he trained Sadie to alert him with a loud ringing noise if someone calls, or to yell “WHO’S THERE?” when anyone knocks on the door. If the fire alarm goes off, Sadie goes off. If Eggers leaves the faucet running, Sadie makes sounds like a waterfall until he turns it off.

Eggers got a service-animal bus pass for Sadie and began taking her everywhere. (He has special insulated cage panels to keep her warm in winter.) For years, few people objected. Then, in the spring of 2007, Eggers went to have his teeth cleaned at the St. Louis Community College dental-hygiene school, and officials there told him that Sadie wasn’t allowed inside because she posed a risk to public health and wasn’t really a service animal. “All I can say is, they were lucky I had Sadie with me to keep me calm when they said that,” Eggers told me.

He filed a complaint with the United States Department of Education’s Office of Civil Rights (O.C.R.), which initiated an investigation. Its conclusion: the school wrongfully denied access based on public-health concerns without assessing whether Sadie actually posed a risk. (Several top epidemiologists I interviewed for this article said that, on the whole, birds and miniature horses pose no more risk to human health than service dogs do.)