Early one summer morning, as rain is misting the foothills of the Blue Ridge Mountains, a middle-aged man is courting a crane. Chris Crowe, 42, bends forward in a slight bow and then ﬂaps his arms slowly, like wings. "Hey, girl, whatcha think," he coos.

Walnut has heard that line before. The stately bird ignores Crowe, reshuffles her storm cloud–gray wings, and snakes her head gracefully to the ground, looking for something tasty to eat.

"Come on, now," Crowe says. The zookeeper grabs a fistful of grass and tosses it into the air. This is Crowe's sexiest move — a sly reference to building a nest together. Walnut looks up, curiosity glinting in her marigold eyes, but then she returns to probing the soft, wet ground with her bark-colored bill.

"Try getting in the van," Crowe calls to me. I follow his suggestion, and almost immediately, Walnut starts responding to Crowe's overtures. She returns his bows and then turns away from him and holds her wings loosely away from her body. Kneeling behind the bird, Crowe rests a hand gently on her back. Then he starts rubbing her thighs, rhythmically, almost pornographically. Thirty seconds elapse before Walnut steps away from Crowe, fixes a few out-of-place feathers, and then stretches out her wings, asking for another go-round.

In past years, Crowe would have taken this opportunity to inject Walnut with a syringe of crane semen. Alas, a matchmaker in Memphis — the keeper of the white-naped crane studbook, whose job is to ensure a genetically diverse captive population — has decreed that they don't need any more babies from Walnut, at least not this year. But that doesn't stop Crowe and Walnut from going through the motions all summer long, five days a week, sometimes several times a day. "It's not exactly fun for me, but it keeps Walnut happy," Crowe says.

More to the point, this strange cross-species seduction has helped ensure that white-naped cranes continue to exist.

Walnut arrived at the Smithsonian Conservation Biology Institute (SCBI), an endangered-species breeding center in Front Royal, Virginia, back in 2004. She was the most genetically valuable white-naped crane in captivity. At 23, she had yet to produce a single chick, and she had a reputation for murdering her mates. Two male cranes that made amorous overtures toward Walnut had been found dead, with their bellies sliced open by her sharp claws. That, at least, was the rumor.

Walnut hatched on July 2, 1981, in an old horse barn at the International Crane Foundation (ICF) in Baraboo, Wisconsin. Her parents, Mercury and Amazon, were wild-caught birds. Captured illegally in China and smuggled to Hong Kong, the two cranes were probably en route to a private menagerie, or perhaps a taxidermist. Instead, they were intercepted by local authorities and eventually shipped to the ICF.

This kind of poaching is less common today, but the white-naped crane population is falling because of a more relentless foe: booming human populations overtaking, polluting, or draining the wetlands the birds need to survive. In addition to demanding as much as several hundred acres of wilderness for a single pair of cranes, these difficult birds seem drawn to marginal places. For instance, one of the white-naped cranes' most important wintering grounds is the 2.5-mile-wide demilitarized zone between North and South Korea.

As conservationists work to persuade people to preserve land for cranes, zoos are pursuing a parallel strategy. They are breeding captive white-naped cranes, creating an "insurance population" ready to be reintroduced should their wild counterparts disappear. But the problem with captive populations of animals is that they tend to get inbred, which is why — in 1981 — keepers were thrilled to have the genes of Mercury and Amazon to add to the mix.

The two wowed ICF staff with their exuberant courtship displays — running, bowing, leaping, and trumpeting their love for each other all summer long. That year, they produced nine chicks, including Walnut.

These wobbly baby birds were raised in small herds, minded by so-called chick mamas who fed them, cleaned their pens, and took them out to a horse ring for daily exercise and to a baby pool for swimming lessons. The chick mamas were mostly volunteers, says Joan Fordham, a former ICF employee. Fordham's 10-year-old daughter was a chick mama one summer, and she apparently didn't get a lot of training. "If the cranes started fighting, she knew how to separate them, and that was pretty much it," Fordham recalls.

The danger of hand-rearing crane chicks, however, is the possibility that they may "imprint" on humans. When it's time to find a suitable mate, some human-imprinted cranes seek out a partner that looks like their presumed parent — a human, instead of another bird. This, it seems, is what happened with Walnut.

Bridging a 320 million–year evolutionary divide, Walnut likely made some human fall in love with her. This relationship, however well-meaning, doomed Walnut to a life of loneliness. Cranes, which mate for life, are happiest when housed in pairs. Walnut was transferred to the Denver Zoo and then the Cincinnati Zoo, but keepers were never able to find her a mate.

In October 2004, Walnut was transferred to SCBI for one last chance at motherhood.

The Smithsonian Conservation Biology Institute sprawls across 3,200 acres of rolling hills near Shenandoah National Park. There, far from the noisy crowds that pour through its sister institution, the National Zoo in Washington, D.C., members of 21 threatened or endangered species are busy having babies, making their kinds a little less rare.

As winter shaded into spring, Carol Hesch, assistant curator at the Memphis Zoo and keeper of the Association of Zoos and Aquariums' white-naped crane studbook, consulted a genetic database and determined that Walnut should be bred with Ray, a comely male crane two yards over.

Ray, however, was already paired with Abigail; if they tried to put Walnut and Ray together, feathers would fly, so to speak. So, the plan was to use artificial insemination, a stressful procedure that requires capturing the birds and holding them down for minutes at a time.

One day that March, Crowe and Warren Lynch, a fellow zookeeper at SCBI, entered Walnut's cage and herded her into a corner. The moment Walnut looked away, Lynch grabbed her body underneath her wings. Then, he stepped over the bird and held her between his legs, facing backward. Kneeling behind her, Crowe began massaging Walnut's cloaca — an all-purpose orifice that birds use for defecation as well as reproduction — and applied gentle pressure to her back, mimicking the weight of a male crane. Walnut purred, her cloaca opened, and Crowe injected semen that the two zookeepers had collected, in much the same way, from Ray.

A few weeks later, Walnut laid two fertilized eggs, which Crowe stole and placed in Ray and Abigail's nest. That was necessary because, in the wild, cranes take turns incubating their eggs, and Crowe didn't want Walnut to shoulder that burden alone. "It's just too much for her," he says, "and I don't want to sit on it myself."

Plus, it was becoming clear to Crowe that Walnut did not see herself as a crane and might not recognize baby cranes as her own. In fact, one young male crane with a clear view of Walnut was always trying to get her attention, with head bobs and calls. Walnut wasn't having any of it.

That summer, however, Crowe noticed that Walnut seemed interested in, well, him. When Crowe stopped by her yard, she would bow her head and raise her wings — motions that Crowe now recognizes as the first moves of a mating dance. "At first, I thought that she was just excited to see me," Crowe says. "But then I'd see the other pairs doing the same things, and it kind of dawned on me."

Crowe accepted Walnut's invitation to dance. Though he felt a little silly, he bobbed his head when Walnut bobbed hers, and raised and lowered his arms like wings. The two circled each other, and sometimes Walnut would make a loud, trumpeting call — the beginning of the white-naped crane love duet. If no one was around, Crowe would try to do the male part of the song — making a Homer Simpson–like "woo-hoo" — but Walnut never found his efforts satisfactory.

As the weather cooled, so did Walnut's ardor. But in the spring, Walnut began greeting her keeper with bows again. This gave Crowe an idea: If Walnut thought he was her mate, maybe Crowe could make that year's artificial insemination less stressful for both of them. "If we could get her able to do it without catching her, there's no stress, no risk of injury," Crowe says. "It's much better for us and for the crane."

So Crowe started training her to tolerate his touch. Reaching his arm out slowly, he'd graze Walnut's tail feathers with his fingertips and then immediately reward her with a dead mouse, Walnut's favorite treat. Eventually, Walnut allowed Crowe to pet her entire back, and she seemed to enjoy it, purring like a contented cat.

One day, after some back petting, Walnut turned away from Crowe, extended her wings, and raised her tail — an invitation to mate. Walnut was asking Crowe to flutter to her back and perform what's poetically known as a "cloacal kiss." Crowe recalls being both startled and amused. "It's what I had been working toward and hoping for, but it was still surprising when it happened," he says. Composing himself, Crowe put a hand on Walnut's back and then rubbed her thighs, going through the motions of artificial insemination.

Once the two got their routine down, in March 2007, Crowe and Lynch captured Ray and collected semen, and then Crowe injected it into Walnut by himself, with her willing participation. Walnut and Crowe would go on to produce five chicks this way — a major boon for the captive white-naped crane population, Lynch says. That also means Walnut's rare genes are no longer in danger of dying with her.

Crowe is proud of his groundbreaking work with cranes, but he admits that sometimes the whole project feels futile. White-naped cranes are continuing to decline in the wild, and many other species appear headed toward extinction. The heart of the matter, it seems, is that most people don't share Crowe's conviction that animals are thinking, feeling individuals. That they have just as legitimate a claim on this planet as we humans do.

How do we bridge the ever-widening gap that separates us from the environment, other animals, and even one another? Maybe, Crowe and Walnut are demonstrating — and my apologies to the Beatles — that all you need is love. After all, what ultimately drives even the most scientific, tough-minded conservation efforts is an irrational, bighearted embrace of a wilderness that's coldly indifferent to us, an impossible connection we feel to animals that we can never truly understand. All this time, we heavy-headed primates assumed our brains held all the solutions, but maybe our hearts can be a better guide.

Excerpted from an article that originally appeared in The Washington Post. Reprinted with permission.