It’s not just day-trippers who are flocking to Boston’s scenic Harbor Islands. Furry four-legged creatures are also swimming or scurrying their way onto the island shores, creating an unexpected population boom.
An ongoing survey of mammals on a half-dozen of the close-in islands has found thriving populations of coyotes, foxes, and deer. Researchers also have seen “aberrantly large” varieties of usually tiny animals — white-footed mice on Peddocks Island, for example — that weigh twice as much as their mainland cousins.
The data, though still preliminary, reveal “dramatic shifts in mammal populations within a given decade,” said Marc Albert, who manages the islands’ stewardship program for the National Park Service.
The animals are arriving by various means, said Lauren Nolfo-Clements, a Suffolk University biology professor who is leading the survey, the first of its kind on the park’s 34 islands and peninsulas. Some were brought by humans, deliberately or accidentally. Large animals are able to swim from the mainland.
Some may cross the water in winter, when parts of the harbor freeze, and for about 15-20 minutes a day, a land bridge appears at low tide between the town of Hull and Bumpkin Island. Park rangers have seen coyotes run across, Nolfo-Clements said.
Animals are often drawn to areas humans frequent, she said, because food is readily available. And on the islands, they lack natural predators, though coyotes and foxes could change that.
Nolfo-Clements and Albert said there has been no conflict between the animals and human visitors, who numbered 484,000 last year, up from 340,000 a decade ago.
But they warned that the presence of rodents and deer can be a hazard to humans. Newly hatched ticks live on small mammals for a time but need to feed off larger animals to reach maturity. Fully grown ticks can carry Lyme disease.
Nolfo-Clements also worries that visitors will not use common sense and try to approach animals to photograph or feed them.
“You’re not in Disney World,” she said.
Nolfo-Clements, known to students as Dr. No, said some of the data has been startling. She has been surprised to find gray foxes — rarer in populated areas than the familiar red fox — living “all over the islands,” and dozens of deer living part time on Grape Island, in Hingham Bay.
In addition to the oversized white-footed mice, Nolfo-Clements has also found meadow voles on Bumpkin Island that are larger and darker in color than usual. The oversized rodents are likely a result of “island syndrome” — the tendency of small animals to get larger when isolated on islands, she said.
Growing deer populations on the islands probably are related to a rebound that has been going on across the Northeast for decades, she said, as the animals recover from being hunted nearly to extinction from the 1800s into the middle 1900s.
Aiding in data gathering is Sophia Bass Werner, 20, an environmental science major at UMass Boston hired through the National Park Service Science Internship Program.
On a recent afternoon, Bass Werner searched for evidence of mammal activity on Peddocks Island, home to Fort Andrews — which held Italian prisoners during World War II — and a small community of summer cottages.
Walking from the beach toward a wooded area, she encountered Catherine O’Brien, a part-time island resident, and asked if she had seen any mammals nearby.
“There were deer tracks on the beach the other morning,” O’Brien said.
What about coyotes?
“The coyotes, I think they’re gone,” she told Bass Werner. “I’ve seen snakes and everything — land snakes — but no coyotes.”
Each week, Bass Werner rotates among six of the islands to place a set of four cameras that capture images of passing animals. She also set out four inked plates to capture tracks.
Walking Peddocks Island, she spotted a break in high grass lining a footpath, descended through a cloud of mosquitoes, and paused about 15 feet down.
She pulled from her heavy backpack a plastic casing, painted with a bark-like pattern, that contained a camera and a motion sensor, and affixed the object to a tree about 2 feet off the ground. She clapped her hands to be sure the sensor was working, then inked and placed the track plates.
After placing the cameras and plates each week, Bass Werner moves to another island and camps overnight, taking observational walks at dawn and dusk, when animals are more likely to be active and visible.
“I actually saw the first fox on Grape Island, from what I heard from the caretaker,” she said. “I really want to see a coyote family . . . but they tend to want to stay away from humans.”
It is still early in the mammal survey, Nolfo-Clements said, and there can be large variations from year to year, so she plans to gather data for “at least a decade” to have enough for analysis.
Albert, of the Park Service, said the information will help park administrators plan for the future.
“Step one in protecting the park resources is to understand what resources we have,” he said. “This is sort of an extended inventory so we get to square one of what mammals are living in the park.”Jeremy C. Fox can be reached at firstname.lastname@example.org.