FORT ROSS STATE HISTORIC PARK — Russian fur traders settled at Fort Ross on the rock-studded California coast in 1812, felling a grove of towering redwood trees for lumber to build a fort, homes and a church.

More than two centuries later, the fort is a state park less than 100 miles north of San Francisco, and the redwood grove has regained the shady, canopy feel of old-growth forest, with a fern-bedecked floor and a creek purling beneath.

But is this habitat close enough to old growth for the marbled murrelet, a quirky little seabird the size of a robin that comes ashore each year to lay an egg on a large, high branch deep in the redwood forest?

Researchers are trying to answer that question by using advanced technology, including artificial intelligence, to more easily locate the elusive birds, whose numbers have declined significantly in the region.

The bird looks for “an old-growth tree with a large-diameter branch with accumulated debris or moss built up on them,” said William O’Brien, who is a wildlife biologist with North Coast Resource Management, a consulting company conducting the study. “Nesting sites in the canopy are protected from high winds and extreme thermal ranges. They prefer nesting sites that are dark and moist.”

The marbled murrelet, nicknamed mamu, is the only seabird that nests in the forests of the Lower 48 states.

When the urge to breed strikes in the spring or summer, a murrelet flies 30 to 40 miles or more inland. The parents take turns sitting on the single egg. When the chick hatches, they ferry food back and forth, taking turns flying to the ocean to fetch fish for their offspring, up to eight times a day.

After a month or more, the adults stop coming, and the chick pulls off its downy coat to expose its adult feathers. Somehow the fledgling instinctually manages to find its way through the dense forest canopy to the ocean — with nary a practice flight. Much about how the birds accomplish this remains a mystery, leading to their description as an “enigma of the Pacific.”

Loggers who spotted the birds called them fog larks, because they were often seen zooming apparition-like through the cloud-shrouded crowns of the redwoods.

The murrelet is listed federally as a threatened species in the Lower 48 states, where there are about 23,000 birds. In California, Oregon and Washington, the bird is considered to be endangered, a designation signaling a more perilous situation. In Alaska, there are healthy populations.

Scott Pearson, a senior research scientist with the Washington Department of Fish and Wildlife, has been doing sea surveys of murrelets for 20 years as part of a team monitoring the health of old- growth forests. He said that the murrelet’s decline is worst in northern Washington, where the number of birds has dropped to roughly 4,500 from 10,000 just two decades ago.

The collapse of some fish populations, such as eulachon — a nutritionally oil-rich fish — is a major contributor to the seabird’s dwindling numbers, Pearson said.

Some protection for the murrelet is now in doubt. The Trump administration is proposing a change under regulations of the Endangered Species Act that would eliminate consideration of harm that refers to dangers to animal or plant habitats. Government officials contend that the existing regulations have been too broad; they say the original law prohibited the taking of an animal, like killing or capturing, but was not meant to include habitat modifications. The move has been viewed as part of President Donald Trump’s efforts to increase drilling, logging and development in the United States and to get rid of regulations that businesses complain have slowed permits.

If that change is adopted, the government would no longer recognize that saving habitat is a way to protect an at-risk species, a long-standing safeguard against the extinction of species.

“Not including harm as habitat destruction would be very detrimental to the marbled murrelet, because they need protection for their old-growth forest home,” said Noah Greenwald, of the Center for Biological Diversity.

Reached for comment, the U.S. Fish and Wildlife Service would only point to the proposed changes and declined to elaborate.

In some places, murrelet numbers appear to be stable, federal wildlife officials stated in a report. But the same review by the U.S. Fish and Wildlife Service, released in December, concluded that the seabird should remain protected under the law.

And a recent decision by the 9th U.S. Circuit Court of Appeals upheld a lower court’s ruling that said the logging of old-growth trees in Oregon’s Elliott State Forest would harm the endangered murrelets and violate the Endangered Species Act.

In Oregon, researchers are conducting two long-term studies that have cast light on the ecology of the reclusive birds. They have found that mamu like to nest among their fellow murrelets, which is important to know for protecting forests. “Young birds go out looking for their habitat and listen for more experienced birds to settle near them,” said Matthew G. Betts, a professor of landscape ecology at Oregon State University who studies the birds. “It results in murrelet communities across the landscape.”

And they have found that the birds shy away from nesting near the borders where the forest meets roads or logging clear-cuts, in an effort to avoid predators. “We’ve seen red tail hawks on camera coming in and eating chicks,” Betts said.

As the bird’s preferred world shrinks, research at Fort Ross is part of an effort to identify and protect other habitats for the bird. The Save the Redwoods League, a nonprofit, owns the Sonoma Coast Redwoods, the grove near the park, and it is funding research into whether, or to what extent, the birds will occupy trees that are developing second growth.