A pair of bald eagles is nesting on the San Francisco Peninsula for the first time in nearly a century, a milestone for the once-endangered symbol of strength and grandeur.

The nest is tucked deep in a fir tree on the far edge of northern Crystal Springs Reservoir — hard for humans to see, but with a bird’s-eye view of 7.15 million noisy neighbors.

“It feels really good. They’re unmistakable — white head, white tail,” said George Chrisman, of Burlingame, who first spotted the birds during a Saturday morning scan of the reservoir two weeks ago, then watched for four hours until he saw the nest.

The last reported nest of bald eagles in San Mateo County was in La Honda — in 1915.

A binocular brigade gathered quickly Friday morning in response to a Thursday night email alert by the San Mateo chapter of the Audubon Society.

The club will host visitors at the best viewing spot this weekend, if it is not raining.

It takes patience, a powerful telescope and a bright morning sun to glimpse the nesting bird’s white head and yellow beak in a lichen-encrusted tree, but Friday’s crowd was exuberant.

“This is so exciting. It’s the first time seeing a nest here in my lifetime. I was afraid I wasn’t going to make it,” said San Mateo resident Larry Caughlan, 63, as commuters rushed by on Interstate 280.

Allan Wofchucky, 75, just off a plane from a birding trip to the Pitcairn Islands, rushed to the reservoir. “Here they are, nesting in our version of New York City’s Central Park. This place is a real treasure.”

They are likely the grandchildren of birds introduced to the Big Sur region in conservation campaigns that started in the early 1980s. At their lowest point, only 35 breeding pairs were counted in the state, and their future was in doubt.

The nest is small, by eagle standards: an estimated 4 to 5 feet across, and 1 foot deep. Eagles reuse their nests every year, and the female is young, so this is likely her first, Chrisman said. They take turns on the nest, he said, rarely leaving it alone for more than a few minutes.

The nest was completed two weeks ago, he believes. This means that the eggs — if fertile and healthy — should hatch within two weeks.

It lofts above an arm of San Mateo Creek, known to have spawning trout. Because it is on San Francisco Watershed property surrounded by fences and guarded by rangers, the nest is presumed to be safe from humans.

Bald eagles once thrived in the Bay Area. While visiting Santa Clara County in 1855, James G. Cooper described “a nest of this bird large enough to fill a wagon, built in a large sycamore tree, standing alone in the prairie.”

After the eagle population collapsed, conservationists with the Santa Cruz Predatory Bird Research Group and the San Francisco Zoo began importing birds and eggs from Vancouver Island and northeastern California in the late 1980s.

That had a big effect, as did the banning of DDT in 1973 and the protection of endangered species.

Bald eagles are again flourishing and no longer need the protection of the federal Endangered Species Act. At last count in 2006, the number of nesting pairs in the lower 48 states had rebounded to 9,700. State protections remain in place.

The Central Coast of California has 21 pairs of nesting eagles, producing almost 200 offspring, according to Glen Stewart of the Santa Cruz group.

Statewide, numbers have surged to more than 200 nesting pairs, according to Dale Steele of the California Department of Fish and Game.

“We’ve created a lot of good habitat and protected them well enough,” said Kathy Hobson, aviculturist at the San Francisco Zoo.

“We’ve been waiting for them,” she said, “and they’re finally making their way back.”