They poop on the lawn. They poop in the park. That poop makes its way into the waterways. Officials in one California city have decided that enough is enough.

In Foster City, Calif., as in much of the United States, the Canada goose population is booming, and the birds are making a mess. Now the city is saying it may have no choice but to cull them in an attempt to reduce potential risks to the public from the birds’ feces.

“We all learn to be tolerant and to coexist with the wildlife, but lately we have been uncovering health hazards,” said Richa Awasthi, the mayor of Foster City, which winds around a lagoon about 22 miles south of San Francisco.

Of the poop, Ms. Awasthi added, “it’s everywhere.”

The proposal, however, has upset some residents and animal rights activists, who fear that the city will round up the birds and euthanize them. Last month, demonstrators let out a rallying cry at a protest: “Let the geese live.”

Thousands have signed a petition calling for a more humane solution. “Any method where they’re not in pain, go for it,” said Erik Allen, 37, who lives in San Rafael, about 40 miles north of Foster City, and who organized the demonstration.

“People just need to understand that animals do the same things that we do,” he added of the goose’s corporeal functions. “Step over it. Care less.”

Foster City has emerged as the latest battleground in America’s war against Canada geese, one that is being waged largely because efforts to save the species have exceeded beyond expectations.

The Migratory Bird Treaty Act of 1918 set the birds on a path from the brink of extinction. By the late 1960s, however, the population had again dwindled, prompting programs like Operation Mother Goose, in which wildlife officials carefully relocated fragile nests and eggs to safer habitats, sometimes by helicopter. There are now millions of resident — or nonmigratory — Canada geese in the United States, according to the Agriculture Department.

The rehabilitation programs were “far too successful,” said Philip Whitford, a professor emeritus of biology at Capital University in Columbus, Ohio.

Resident Canada geese, Dr. Whitford said, prefer short, fertilized grass and easily accessible bodies of water. In many American cities, he said, “we’ve created the perfect circumstances for them.”

Now, the birds, once a symbol of the North American wilderness, have become a nuisance. They have downed planes and colonized golf courses. They can be aggressive when they feel threatened. Then there’s their extraordinary capacity to defecate: An adult bird can drop about a pound of feces each day.

In Foster City, which counted 323 resident Canada geese last year, up from 181 in 2020, the debate over what to do about the waterfowl has raised questions about how people share urban spaces with wildlife as their natural habitats have been decimated. The debate has also exposed divisions among residents of the city, which, like much of the Bay Area, leans liberal.

The city has tried frightening the birds away with dogs and strobe lights. It has punctured their eggs and covered them with oil to prevent them from developing. It has erected fences and sprayed grass with a liquid that is repulsive to geese. According to a mitigation plan, some of those methods failed, while others had only limited effect.

Though the exact public health consequences in Foster City are unknown, high bacteria levels in the water, “partly attributed to goose droppings,” have resulted in closure of some of the town’s beaches, according to a notice on the city’s website.

Some residents say that dodging the droppings has become a routine part of using the city’s parks and footpaths. In an email to the City Council, one woman said that her family showers immediately after kayaking to avoid bacteria. A man said his 2-year-old daughter became ill after putting goose feces in her mouth.

Raju Gadiraju, a biopharmaceutical executive, said in an interview that he no longer lets his dog run off its leash because the dog “likes to eat geese droppings.” He added, “it’s just disgusting.”

The city has applied for federal permits that would allow it to cull its goose population. (A common method involves euthanizing the birds in a portable gas chamber.) Ms. Awasthi, the mayor, emphasized that the city had not made a final decision about whether to cull the birds, or what method it might use.

One challenge is that while culling the birds instantly reduces their number, populations often recover quickly if there are more geese nearby, said Paul Curtis, a professor of wildlife science at Cornell University.

“There is no permanent solution,” he said. “I think we’re going to have to learn to live with them as best we can.”

The discussion has upset some Foster City residents, like Michael Schultz, 37, an accountant. “It seems really inconsistent with California and Bay Area values,” he said in an interview, adding: “Is this really necessary?”

But others have urged the council to ignore the protests, which they perceive to be driven by a small group of outsiders with an agenda.

“I urge all of you to not let a FEW DOZEN people dissuade you from doing what is best for all of Foster City,” one person wrote in a letter to the city. The goose droppings, the person added, were “unsightly, unhygienic and frankly unacceptable.”

For Mark Beltran, another resident, controlling the goose population — even if it means euthanizing them — seems like a reasonable measure. “The birds have just taken over,” Mr. Beltran, who works in corporate finance in Silicon Valley, said in an interview.

“This beautiful place that we call home, we cannot even use it as it was intended,” he added. “I’m not here to kill birds. I’m here to save our local environment.”