BIRD WHISPERER: The endangered mangrove hummingbird is one of the subjects of Jared Flesher’s documentary “Field Biologist,” being screened Saturday at Princeton Public Library as a special event of the Princeton Environmental Film Festival. Mr. Flesher followed Hopewell Township resident Tyler Christensen in his travels through Costa Rica, where he found rare species.

While shooting the documentary Sourlands a few years ago, filmmaker Jared Flesher was having trouble getting some needed shots of birds in the forest. A friend suggested someone he thought would be able to help.

“He said, ‘You have to meet Tyler Christensen, he’s the best,” recalled Mr. Flesher, who has screened Sourlands and The Farmer and the Horse at previous Princeton Environmental Film Festival (PEFF) showings. “I got in touch with him and we went into the woods together. Sure enough, he found birds that would otherwise have been really hard to see. We got some great shots.”

A friendship developed between the two men, and inevitably, another film. Field Biologist, Mr. Flesher’s account of Mr. Christensen’s bird-researching venture to Costa Rica, will have its New Jersey premiere this Saturday night at the Princeton Public Library. At the screening, a special event of the PEFF, the two men will be on hand to answer questions after the 7 p.m. showing in the library’s Community Room.

Mr. Flesher, who is 31 and lives in East Amwell Township, followed Mr. Christensen, 22, a resident of Hopewell Township, through travels to the cloud forests of Monteverde and the mangrove swamps of the Nicoya Peninsula. In the film, Mr. Christensen comes up with a plan to try and help save the highly endangered mangrove hummingbird.

Talking about his colleague last week, Mr. Flesher was clearly enthusiastic. “This is a guy who loves the natural world,” he said. “When you go out into the woods with him, he’s an incredible communicator. He finds birds, bugs, frogs, and snakes, and he can tell a really interesting story about everything he finds.”

The fact that Mr. Christensen failed biology in high school is an irony not lost on Mr. Flesher. “Tyler wasn’t a great student. He didn’t really know how he was going to put his passion for nature into a career,” he said. “But he got really interested in studying birds. He just kind of traveled a bit, and did some rock-climbing. He was working at a couple of places in New Jersey doing bird-banding. He eventually decided to pick up, one winter, and head down to Costa Rica, and he started doing his own research on migratory birds. No background, no degree — just amazing passion.”

Field Biologist has dual themes. “There’s the age-old question of ‘What should I do with my life,’ and finding your way,” Mr. Flesher said. “And then there is the bigger story, which is that scientists say we stand to lose 50 percent of the species on earth in this century due to habitat destruction, climate change, and over-harvesting. There is definitely a bio-diversity crisis. My goal is to bring up some of these issues and tell the story through Tyler’s eyes, focusing not just on the problem but on why we should bother to save what we have left. It’s ultimately a positive story about the natural world.”

Mr. Flesher trailed Mr. Christensen in Costa Rica two winters ago. “We spent eight weeks there,” Mr. Flesher said. “The plan was ‘wherever he goes, I go.’ So I just followed him. We were chasing incredible birds, snakes, and butterflies. It was really special, which I hope comes through in the film.”

Susan Conlon, Princeton Public Library’s Youth Services Team Leader and the librarian in charge of the PEFF, said the showing of Field Biologist this weekend is part of an effort to broaden the popular annual event, which takes place in January. “We want to expand the reach of the PEFF and continue to screen even more films year-round,” she said in an email. “It is also exciting to have the opportunity to present new films like Field Biologist, and for the film to have its New Jersey premiere with us, with Jared and the film’s subject, Tyler Christensen, here for a question and answer session.”

Admission to the screening is free. Visit www.princetonlibrary.org or call (609) 924-9529 for more information.