IDYLLWILD — The night is cold, dark and silent in Tahquitz Valley, a bowl-shaped expanse of boggy meadows and sloping forests 8,000 feet above sea level in the San Jacinto Mountains. Suddenly, the "chirp, chirp" of an electronic bat detector is joined by the sound of footsteps trampling the undergrowth.
Drew Stokes strode through knee-high ferns, the beam of his headlamp fixed on a bat caught in a net and baring its razor-sharp upper canines. "It's a long-eared myotis, a male," Stokes said as he disentangled the creature, then methodically recorded its vital statistics in a notebook. "These guys are really cool," he added, admiring the bat's translucent wings, glossy fur and black ears.
"All done," he said with a smile, releasing the bat back into the night sky.
Stokes was part of a team dispatched to the area just south of Palm Springs by the San Diego Natural History Museum. Their mission: to inventory Tahquitz Valley's life forms from stream bed to treetop, then compare the findings with those recorded a century earlier by Joseph Grinnell and Harry Swarth of UC Berkeley's Museum of Vertebrate Zoology.
The earlier expedition's notes, photographs and specimens established a benchmark for gauging changes in Southern California's biological landscape.
"This value will not, however, be realized until the lapse of many years, possibly a century," Grinnell wrote at the time, "assuming that our material is safely preserved."
The 10-member San Diego team set out at dawn one day in late June, completing an arduous three-mile hike from Idyllwild to Tahquitz Valley in about three hours. Seven horses and a mule laden with supplies and equipment followed close behind.
Working out of pup tents and a battered cabin, the team -- led by Phil Unitt, curator of the museum's department of birds and mammals -- spent seven days tramping through wet meadows and forests of Jeffrey pine and white fir, many of which were already mature when Grinnell and Swarth camped in the area in 1908.
They identified birds and collected feathers from which chemical constituents would be extracted. At night, they searched for amphibians, owls and night-prowling mammals. Some mammals and birds -- none of them threatened or endangered -- were captured, skinned and prepared for storage and study in museum archives.
On tree branches, they placed traps baited with strawberries and peanut butter in hopes of attracting a flying squirrel. The small, nocturnal, arboreal creature, able to glide 100 feet, has not been seen in the San Jacinto Mountains since the late 1970s.
But most of their work focused on determining range expansions and retractions of common species, from chipmunks to chickadees.
For example, Grinnell and Swarth noted an absence of robins, which today are among the most common birds in the valley.
"What's up with the robins?" Unitt asked one afternoon while eating a flour tortilla filled with peanut butter and jelly. "The robin has clearly moved in over the past century. Why?"
An hour later, Unitt was back roaming the wilderness, taking copious notes of everything he saw and heard. Peering through binoculars, he scanned a marsh, bringing into focus a tiny brown bird darting in and out of a tangle of vines and tree limbs.
"How perfect can it get for a Lincoln's sparrow?" he said, cracking a smile. "This is the southernmost nesting site of this species anywhere."
Half a mile away, herpetologist Brad Hollingsworth followed a stream, inspecting every inch of ground for signs of the federally endangered mountain yellow-legged frog, a species common in the area only a few decades ago.
No luck that day.
"But not finding a particular species is also important information," Hollingsworth said. "We're chronicling environmental changes and raising new questions. The answers may help document the big smoking gun in the room, which is global climate change."
Grinnell and Swarth would not have argued with any of that, said team member Chris Swarth, grandson of Harry Swarth and manager of the Jug Bay Wetlands Wildlife Sanctuary in Maryland.
"We're retracing the steps of two characters who were on the ground floor of what the field of ecology has become today," he said.
Hanging on every word was Swarth's 18-year-old son, Evan, who wants to become a marine biologist.
Earlier, Evan and two other teenagers on the team had used a portable ladder to peek inside six nesting boxes that U.S. Forest Service biologists had nailed onto trees four years ago in hopes of attracting flying squirrels. They found no tenants, however, other than mountain chickadees.
Ten traps that mammalogists had baited with strawberries and peanut butter also failed to attract customers. "The fate of the flying squirrel in the San Jacinto Mountains," Unitt said, "remains veiled in mystery."
Overall, the expedition was a success. The team identified 61 bird species, the same number logged by Grinnell and Swarth. "However, we had 16 species they didn't," Unitt said, "and they had 16 species we didn't."
"Our bat list was phenomenal," he said. "We captured and released eight species, including a rare one known as the little brown bat. We also identified seven additional species with electronic detectors, including the yellow bat, which roosts six miles away in the skirts of palm trees."
Motion detection cameras snapped photos of gray foxes, mule deer, bobcats and ground squirrels. Stokes also found a lone yellow-legged frog in a meadow stream.
"We hope what we learned will be of value for another century," Unitt said, "when we can't possibly imagine what Southern California will be like."