The tiny, thin bird darted from oak to juniper trees in the Santa Rita Mountains, uttering soft “whip” calls, catching insects and occasionally flying over a birdwatcher’s head.

That was the pine flycatcher’s public intro-duction to Arizona and the United States. The Mexican bird, very hard to identify, was recorded in this country for the first time at a remote, rarely visited spring area in a grove of lush woodlands in late May.

For dedicated Arizona birders, this flycatcher, 5.5 inches long, “has been the Holy Grail ... for decades,” observed Kenn Kaufman, author of birdwatching guides and a former Tucsonan, in a Facebook post. Many ornithologists and other bird experts have suspected for years that it could occur here, but it had never been confirmed.

A veteran Tucson birdwatching guide, David Stejskal, first reported the flycatcher. On May 28, he spotted and photographed the flycatcher and recorded its voice while camping with family and friends.

It was flitting about at Aliso Spring, in a grove of sycamore and oak trees at about 5,800 feet and close to 10 rocky miles of dirt road west of Arizona 83 near Gardner Canyon.

Using sonograms that trace a bird’s voice with images, he and some colleagues then confirmed its identity. On Memorial Day, Stejskal and six other expert birders returned to Aliso Spring, easily spotted the bird, took numerous photos and got more recordings.

“I was and still am super-excited about this bird showing up in Arizona from Mexico for the first time. Many avid birders like me live for making a discovery like this!” Stejskal wrote in an email Friday. As he wrote, he was riding a small, 22-seater turboprop heading west over the Bering Sea, bound for St. Paul Island in the Pribilofs. An owner of Field Guides Inc., an Austin-based bird touring company, he was leading nine other birders on a tour.

Since May 28, the pine flycatcher has been seen and photographed daily. The female bird has nearly finished building a nest in an oak tree, according to online reports from birders, although no male bird has been seen.

Only female flycatchers build nests, said Laurens Halsey, a Green Valley-based bird guide who saw the bird last week and said “thrill is rather an understatement” of his feelings about the discovery.

Dozens of birdwatchers, from California and New Mexico and farther away have since flocked to catch glimpses of the flycatcher via a road whose last three miles require high-clearance, four-wheel drive vehicles, said Gary Rosenberg, another Tucson birding guide who helped Stejskal identify this bird.

“This is a very significant discovery and a big deal in the birding community,” said Nate Swick, editor of an American Birding Association blog.

New discoveries of bird species in the U.S. are rare, occurring maybe twice a year, said Rosenberg. He and Stejskal sit on the Arizona Bird Records Committee, a group of ornithologists that evaluates new bird records in the state.

That committee and the birding association’s Checklist Committee must review the pine flycatcher record before the bird officially is classified as existing in the U.S., Swick said.

That’s probably a foregone conclusion, but it may not happen until next year, he said. Once that happens, the association will likely classify it as Code 5 — the rarest possible ranking.

The flycatcher is also one of numerous Mexican-based species that have been flying into Arizona in increasing numbers in recent years, said Rosenberg and Tucson Audubon’s Jennie MacFarland. That’s very possibly due partly to the region’s warming climate, they said.

But none of the other species were new to this country. They’d simply been seen more rarely before.

Best-known of these is the highly colorful elegant trogon, long a regular Mexican visitor, said MacFarland, a bird conservation biologist. Its loud, clacking call is heard often on some of the pine flycatcher recordings from the Santa Ritas, long a trogon hotspot.

The flame-colored tanager, slate-throated redstart, rufous-backed robin, tufted flycatcher and rufous-capped warbler all remain “incredibly rare” in the state, but are turning up more often in the Huachuca and Santa Rita mountains and some even in the Catalinas, said MacFarland.

The pine flycatcher, commonly seen in mountains from northern Guatemala to southern Sonora, belongs to the Empidonax flycatcher genus.

Individual Empidonax species look very much alike. Their presence can torment even some experienced birdwatchers who try to tell the species apart.

Empidonax birds, called empids for short, include Acadian, gray, Hammond’s, willow, dusky, Pacific slope and Cordilleran flycatchers, the latter two formerly combined as the Western flycatcher.

Probably the most prominent empid here is the southwestern willow flycatcher, an endangered subspecies that frequents streamside riparian areas.

“Identification (of the pine flycatcher) is subtle — shaped much like a ‘Western’ Flycatcher, but colored more like a dusky,” Rosenberg wrote on Facebook. “The average birder probably wouldn’t recognize it,” he added in an interview.

Indeed, when Stejskal first heard the pine’s call while eating breakfast, he thought it sounded similar to those of the gray, dusky and willow flycatchers, he wrote in his email. Glimpsing it darting from a perch inside an oak tree about 15 feet off the ground, he then thought it a dusky flycatcher, a common Southern Arizona migrant. Since the dusky is usually nesting in Northern Arizona by this time, Stejskal decided to look more closely.

He tracked it down on a hillside as it hopped among oaks and junipers.

“The bird wasn’t at all shy and seemed to be very confiding, a few times flying right into the tree over my head!” he wrote.

He saw it twice fly into a nearby tree where it would land in narrow crotches, sit down and turn to face different directions, which Stejskal interpreted as investigating potential nest sites.

Since dusky flycatchers don’t nest here, that behavior made him think pine flycatcher.

It is very unusual that this bird’s first U.S. record is a nesting individual, Stejskal wrote, adding that he was pretty sure that he heard a pair of them calling when he returned to the site May 30.

But with no males seen since then, he wrote, “It may simply be a lone bird that built a nest in hopes of attracting a mate — which may never show up.”

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987.