Family of gray foxes living in Presidio for 1st time in decade

For the first time in at least a decade, a gray fox family has set up housekeeping in the Presidio — in a roomy storage building near Crissy Field complete with a birthing den for newborns.

The foxes’ images were captured on motion-activated, night-vision cameras set up by Jonathan Young, the park’s wildlife ecologist, after workers reported seeing what they thought were signs of coyote occupation.

Instead of coyotes, Young said, the cameras showed what appeared to be a playful tussle between at least one adult gray fox and two young ones.

“Or maybe it was the other way around,” Young said. “It’s hard to tell the young from their parents at this stage because the kits have already grown close to full size, though they’re still full of play. They must have been born last March or April.”

Young recalled that an adult gray fox, probably a male, fled up a tree in the Presidio in February to escape hungry coyotes, and he wondered if it was one of these foxes. “I hope so,” he said.

“We want to encourage more gray foxes to breed in the Presidio, and the big question now is whether we can manage their habitat so they can coexist with the coyotes without predation,” Young said.

The park’s biodiversity expert is planning how to keep the family safe from the coyotes, which have been increasing in numbers in the Presidio for more than a decade.

The rise in the park’s coyote population may be the reason that gray foxes have been “virtually extirpated,” Young said. The family that has taken up residence, he said, may have migrated north from the Peninsula — or might even have crossed the Golden Gate Bridge from Marin.

“It’s happened before,” he said.

The old military building where the foxes now live backs up against a hillside covered with thorny blackberry bushes that coyotes avoid. So Young plans to make it a safe habitat for fox families, with access through a small section of old, plastic piping just wide enough for a fox’s body but too small for coyotes.

The large room where the foxes have settled has a smallish hole in the floor, and Young believes the kits were born beneath it in a cozy, safe area that wildlife specialists call the birthing den, or natal den.

Meanwhile, one of the tribe must be quite a hunter, for one of Young’s cameras caught the fox with a large rat in its mouth — food for the family, it appears.

Young hopes that visitors walking with smartphones anywhere in the Presidio will log on to an app called iNaturalist, a citizen science crowdsourcing site, and post their observations of any wildlife they encounter.

The iNaturalist site is based at the California Academy of Sciences. “It’s an incredibly powerful tool for people to share nature with each other and help biologists keep track of everything alive in San Francisco,” Young said.

David Perlman is The San Francisco Chronicle’s science editor. E-mail: dperlman@sfchronicle.com Twitter: @daveperlman