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In 1978, Alfred Larson was looking for a hobby that would keep him busy after he retired from his job at a sawmill plant near Boise, Idaho. He remembers reading in National Geographic that captured his imagination—about crafting wooden nests for bluebirds to save them from dizzying declines. Around this same time, he and his wife Hilda welcomed a new guest to their backyard: a Western Bluebird.

“We noticed a bluebird going in and out of a cavity of an old, dead snag,” Larson says. “I thought, ‘Gee whiz!’ I had heard about bluebird trails out East that  had set up. If I put up boxes on my ranch, I’d have a captive group of birds to take pictures of.”

So he got to work, building nest boxes out of pine scraps and board ends from his old sawmill to install around his property. Soon after, he went on a field trip with the and put up another 25 boxes in various habitats. And then even more.

Four decades later, at the age of 96, Larson is monitoring almost 350 nest boxes on six different bluebird trails . From the Owyhee Mountains to Lake Cascade, he and his fellow community scientists peek into the rustic abodes every nine days to band any residents and jot down notes on behavior and growth. Larson organizes the data and shares it with the Cornell Lab of Ornithology‘s .

“I got carried away,” the Golden Eagle ̽����ѡ charter member says. “I settled on a simple design that [was] easy to build and easy to monitor. I kept adding more boxes on these trails, and these birds responded.”

“This year he‘s banded over 900 birds,” says Cathy Eells, a Golden Eagle ̽����ѡ member who often drives Larson out to his trails. “In 40 years, think how many homes he’s provided for parents. Bluebird recovery efforts like Larson’s rose in popularity in the 1970s and ‘80s when people discovered how easy it was to use homemade nest boxes to attract the beloved azure passerines. As a result, over the past several decades nest box and trail building has become one of the largest grassroots conservation activities in North America, biologist Myrna Pearman writes in the .

That, in turn, has been good news for bluebirds. Prior to the big nest box craze, all three North American species—Western, Mountain, and Eastern—, due to “the elimination of dead trees with the invention of gas-powered chainsaws in the 1930s . . . along with the widespread use of pesticides to kill insects,” says bluebird photographer and expert Stan Tekiela. Studies in the 1970s to the death of hundreds of Mountain Bluebird chicks in western Canada.

The birds also faced competition from European Starlings and other introduced species that crowded out their natural cavities. These clashes, coupled with the wane of open fire-managed ecosystems, forced Mountain Bluebirds to —putting them in the crosshairs of Eastern Bluebirds.

Today, populations for Mountain, Eastern, and Western Bluebirds have stabilized to levels of “least concern.” But other threats could lie ahead with climate change and on local land conditions.

“As the climate warms, high-elevation populations of bluebirds and fellow species will lose nesting habitat,” says Heidi Ware Carlisle, education and outreach director at Boise State University's Intermountain Bird Observatory. She notes that ̽����ѡ's 2014 climate report in the Mountain Bluebird’s summer range. Much of that drop off seems to be in Idaho.

“If you look at the [bluebirds’] predicted range map as a whole, it looks like they will do okay. But if you focus, you realize that they are losing almost all their habitat during the most important time of year,” Carlisle says.

But are the projections panning out? To answer this, volunteers in Idaho and other states are surveying bluebirds for ̽����ѡ’s Climate Watch network. Their data, along with stats collected by nest box monitors such as Larson, help experts map geographic shifts in real time. “The thing with global warming is that the habitat has changed dramatically because of people; each species responds idiosyncratically,” says Brooke Bateman, senior ̽����ѡ scientist and leader of Climate Watch.

Regular breeding counts could also show if climate change is rendering the wooden nest box design any less effective. “While nest boxes do provide cavities for bluebirds to use, they are not as insulated compared to a natural cavity and are exposed to extreme and variable temperatures,” says Meelyn Pandit, a PhD Student in biology at the University of Oklahoma. In his experiments to create "the ultimate bluebird box,” Pandit’s found that aluminum foil and polystyrene batts are the best way to stabalize temperatures. “If global warming is not curbed, then we may have to retrofit nest boxes with these insulators to prevent both the adults and nestlings from being exposed,” he says.

For now, however, Larson’s age-old creations are giving bluebirds a much-needed edge in survival. “Al Larson and others have helped boost local populations because they provide the species with additional nesting sites,” Pearman, the author of Mountain Bluebird Trail Monitoring Guide, says. What’s more, the long-term data sets from his trails are critical to scientists trying to prepare for the future. “They are a time capsule that allow us to travel back in time to see what life was like for birds before major impacts from climate change began to show,” Boise State University’s Carlisle explains. “Without Al's past work—and without continued monitoring of his boxes—we won't know how to anticipate and try to prevent population loss from climate change.

Many of Larson’s trail buddies are wary of the day he decides to retire again. Boyd Steele, a volunteer who regularly assists Larson with the nest boxes, says the nonagenarian has been steadily passing down his knowledge. But his devotion to bluebirds will be hard to replace. “I don’t think there’s anybody who is as dedicated as Al,” Steele says.

Filmmaker Matthew Podolsky echoes that sentiment. After being introduced to Larson through a graduate advisor at Boise State University, he and his peer Neil Paprocki tracked the local legend with a camera for weeks. The resulting 30-minute documentary, titled , of course, went on to be nominated for an Emmy Award in 2015.

“I remember the first trip I took out to the bluebird trail with Al, and I was very surprised by the speed with which he moved from box to box,” Podolsky says. “He was hiking across difficult, uneven terrain. Neil and I often struggled to keep up with him. Sometimes we’d check more than 100 boxes and be driving home in the dark.”

Larson’s connection to bluebirds, as chronicled in the documentary, has been life-affirming for Podolsky. “Al is a living example of how much one person can achieve when they set their mind on a task. But he’s also an example of the benefits that a project like this can have for people,” Podolsky says. “[Bluebirds] have given meaning to Al’s life, and they are truly the secret to his longevity.” Just as he, and the countless community scientists like him, are to theirs.

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Interested in bluebird conservation?  or volunteer with the , which is celebrating its 40th anniversary this year.