Juan Oteyza pulled the first of the Florida grasshopper sparrows from the wooden transport box and checked its heart rate, the four tiny bands on its legs and the miniature transmitter strapped to its back.

The year-old bird, born and raised in captivity, was about to head out on a mission that could help save its species and solve the mystery of its decline that has confounded researchers for a decade.

Oteyza, a research scientist with the Florida Fish and Wildlife Research Institute, smoothed down the feathers of his small, brown recruit. “There’s a lot riding on him,’’ he said.

The Florida grasshopper sparrow will probably be the next bird to go extinct in the United States unless an experiment playing out in the spring can turn things around. Just 30 breeding pairs remain, almost all living on a stretch of state grasslands 50 miles south of Orlando.

In the past three years, a coalition of nonprofit research centers, state and federal agencies and environmental groups figured out how to breed a new generation of grasshopper sparrows in captivity. One by one, the birds were released on this prairie over the past two springs to mate with what remains of their species.

Biologists are tracking the sparrows’ every move with the help of the transmitters. They’re watching their courtships, listening to songs for clues on how they are doing, putting two-foot-high fences around their nests for protection and waiting to see the tiny white speckled eggs that would signal success.

“No one’s ever done this before,’’ said Paul Gray, science coordinator with Audubon Florida, one of the partners. “We’re in completely new territory.’’

The sparrow project does indeed chart a new path in biological engineering, which goes beyond traditional conservation measures of preserving habitat, putting laws into place to protect troubled birds and studying their declines. It’s also part of a wider push by researchers to use their growing knowledge of birds — the most studied branch of wildlife — in complex interventions to counteract habitat loss and climate change.

In the Southeast, biologists are installing the reproductions of the cavities red-cockaded woodpeckers need to provide homes for and protect the endangered birds.

Off the coast of Maine, researchers have lured Atlantic puffins to a remote island to rebuild communities of the exotic seagoing birds. Their tools included decoys and mirrors that fooled the first puffins into thinking they were part of an existing colony.

In one of the most sophisticated interventions, scientists in Hawaii are trying to save endangered birds by sterilizing mosquitoes that spread deadly avian malaria throughout the islands.

Most of the projects are still underway, so full results aren’t in. But researchers say creative solutions like these may be the best option for protecting threatened species.

The grasshopper sparrow is the case they are following most closely.

A bold plan

The Florida grasshopper sparrow is a living symbol of the unique grassland called the Florida prairie defining it by its song and presence, and it is part of the state’s avian culture. It sits at the baseline of the balance of nature, eating insects and grasshoppers — hence its name — and in turn being consumed by a variety of predators.

For decades, the small, reclusive sparrow subspecies was in a slow decline as the state’s rapid development consumed 90 percent of the prairie land.

In the early 2000s, the number of grasshopper sparrows across central Florida began a startling drop.

“The populations took a dive,’’ said Mary Peterson, the U.S. Fish and Wildlife Service’s lead sparrow researcher. “We really needed to do something.’’

The list of suspects out to get the sparrow is long — fire ants, skunks, snakes, hawks, heightened flooding and shifting climate. “But we believe something bigger is causing this decline,’’ Oteyza said. “We haven’t confidently identified what that is.’’

The coalition drew up a bold plan, with an annual budget of $1.2 million in private and public funds. Recruited to breed sparrows were two private research centers in Florida experienced in breeding rhinos, monkeys and parrots.

The breeding technique came together through trial and error, amid sharply differing opinions among the many partners. Researchers tried collecting eggs and raising the chicks using hand puppets made to look like birds. That proved unreliable, and eventually they brought in wild sparrows to mate and raise their own offspring.

At the White Oak Conservation Center, a massive tract near Jacksonville, the staff designed enclosures the size of small houses so the birds could breed in a controlled outdoor setting surrounded by fields.

“This is not the Florida prairie, but it’s as close as we can get,’’ said Andrew Schumann, White Oak’s avian collection manager.

At the Rare Species Conservatory Foundation near the Everglades in Palm Beach County, the first sparrows were born to the sound of howling monkeys and squawking parrots.

“That got everybody jumping for joy,’’ foundation president Paul Reillo said. “Then somewhere between three to five weeks after that, these perfect sparrows fell over dead. They were perfect birds. They just fell over dead.’’

The culprit turned out to be a lethal pathogen, and so began a dispute that has followed the project. The pathogen was found both in captivity and in the field, researchers said, but they disagree on whether this is an ongoing threat or a germ that is usually dormant.

Reillo pushed to delay releasing birds for a review. “We should be careful, particularly when we know there’s a pathogen out there,’’ he said. “I’m just saying let science do its job.’’

But federal and state agencies decided there was too much at stake to slow down. They ended the Rare Species contract and kept breeding at White Oak.

The rate of births at White Oak that followed surprised everyone. “They breed like crazy,’’ said Reed Bowman, a biologist at the nonprofit Archbold Biological Station who helps lead the project. “This is a little sparrow that only lives two or three years. They are geared to pump out as many broods as they can.’’

By this spring, the project had produced 250 chicks. For weeks, researchers carted the sparrows in a big wooden carrying case to the prairie and freed them to settle in for the breeding season that will soon be reaching its peak. .

At the crux of it

Whether the captive-bred and the wild birds will mate is not the only question hanging over this project.

The consortium has a 70-page plan for the sparrow, but it doesn’t have a clear answer on what to try next.

“If this isn’t successful, we’ll have to rethink what we’re doing,’’ said Ashleigh Blackford, U.S. Fish and Wildlife’s supervisor of the sparrow recovery program.

Even as the new young sparrows were being released, researchers couldn’t say whether there is a force beyond predators and habitat loss that could doom them. They said they hope they can boost the population and overcome the declines, but the original mystery remains.

Some researchers worry about whether they have waited too long to start the breeding program, with just a handful of the wild birds left. Others wonder whether there is enough public and political support for this kind of expensive and long-running rescue mission.

“Are we really doing our best?’’ Reillo said.

Researchers concede they don’t have all the answers. But they say projects like the sparrow rescue could lead to them. The discoveries can be used on other grassland species, which are facing the steepest declines of any bird categories.

As wildlife agencies learned to manage the prairie with controlled burns and removal of nonnative plants and predators, the environment has improved for all prairie wildlife.

Researchers say they are encouraged: The captive breeding seems to be working. More than 20 of the released birds have been spotted recently doing well out on the prairie.

Some of the male grasshopper sparrows have started singing their mating songs on cue. Three of the captive sparrows have paired up with mates, two with one another and a third with a wild sparrow. They should know by May or June whether there are eggs in the nests.

“We’re right at the crux of it,’’ Bowman said. “I’m as optimistic as I’ve ever been.’’

Correction: This article has been revised to correct the number of birds spotted in the prairie.

Anders Gyllenhaal is a veteran journalist who writes about birds and co-publishes the birding website, FlyingLessons.US.