The summons referred to a two-story brick house in Astoria that Mr. Pomares, a 53-year-old home renovation contractor, had recently refurbished and rented to tenants. Titled “vector control inspection work order,” the citation accused him of violating what appeared to be a paradoxical imperative: “standing water” in a birdbath. The violation of Article 151 of the city’s health code could subject him to a $2,000 fine.

“I bought the birdbath brand new. I thought I was doing something good, and I changed the water every other day,” Mr. Pomares recalled. “I had beautiful birds.”

Mr. Pomares was one of 699 New Yorkers who learned last year that April showers can bring a lot more than May flowers. They can also deliver property owners a summons. Most of those issued in 2011 were for poorly maintained swimming pools and standing water at construction sites. Even an ordinary puddle can lead to a violation. So can a birdbath, as four surprised New Yorkers found out.

During mosquito-breeding season, from April to October, standing water on the ground, in roof gutters, on swimming pool covers and in discarded tires, among other places, can violate Article 151, which covers pest prevention and management. Health officials said the 699 summonses issued last year was about average.

In a city where Mayor Michael R. Bloomberg has all but banned smoking and waged war on soda and trans fats, some New Yorkers may complain that the crackdown on birdbaths is yet another intrusion by the nanny state or a ruse to raise more money for municipal coffers.

Actually, a regulation against stagnant water has been on the books for more than a decade, but in the battle against West Nile virus, the health code was amended last year. It explicitly made landlords liable and applied the rule, apparently more broadly, to “standing water” rather than “stagnant water” and further empowered the department not only to prevent “the breeding or harborage” of mosquitoes, but also to prevent “conditions conducive” to their breeding or harborage.

As a result, one creature’s beverage can be another’s breeding ground. Dimitri Gatanas, an owner of the Urban Garden Center in East Harlem, said, “When there’s a mosquito issue, someone gets nervous and files a complaint.”

One solution is to buy a birdbath that circulates the water. Some nurseries sell pellets that John Razzano of Market Garden Center in Brooklyn says “kill the mosquito larvae, but don’t hurt the birds.”

Mosquitoes that carry the West Nile virus lay their eggs in standing water, although exactly what constitutes standing water is ambiguous. Asked to explain the difference between standing and stagnant, a department spokeswoman, Chanel Caraway, gamely said: “Standing waters become increasingly stagnant with time as they become more and more concentrated with decomposing organic material, which is food for the mosquito larvae.”

Ms. Caraway emphasized that the agency is not necessarily anti-birdbath. “The health department will issue a notice of violation for standing water in a birdbath only if that water is stagnant, not simply for having water in a birdbath,” she said. “The decomposed organic matter found in stagnant water is the food for mosquito larvae. The department recommends replacing the water in the birdbath every two to three days to prevent mosquito breeding.”

The federal Centers for Disease Control recommends that homeowners “replace the water in birdbaths weekly.”

Last year, the city’s health department recorded 11 cases of West Nile virus, including one death, of a Queens resident.

“Enforcing the health code sections that do not allow for standing water is key to preventing the spread of this disease in the city,” said Waheed I. Bajwa, executive director of the vector surveillance and control office. “The standing water violations are issued during the mosquito season to property owners whose property has amounts of standing water and decomposing matter.”

Health officials say they take reports (calls to 311 or through nyc.gov/health/wnv) “to track significant problem areas,” although the department’s Web site does not encourage New Yorkers to turn in their neighbors.

Robert G. Materson of Greenpoint, Brooklyn, believes a neighbor did just that. A 76-year-old retired psychologist, he was penalized with a violation for a birdbath in his backyard that, he insists, he fills with fresh water every few days.

“Between the birds and the evaporation, there’s no water in that sucker after a couple of days,” Mr. Materson said. “I get four blackbirds splashing out all the water to three-quarters of an inch and then the sparrows show up.”

The violation he received, however, said his property was “not kept free of conditions conducive to the breeding of mosquitoes in that murky standing water was observed in a birdbath in the rear yard.”

Last December, a hearing examiner for the city’s Office of Administrative Trials and Hearings weighed the inspector’s testimony and photographs against Mr. Materson’s claim that “the water was neither stagnant nor murky.” The examiner ruled against Mr. Materson, explaining that no matter how conscientious he was ordinarily, he was not present during the inspection and “therefore, he could not have observed what the inspector claims to have observed.”

Mr. Materson is appealing his $350 fine (he successfully appealed an earlier violation after a hearing examiner agreed that the birdbath itself was brown, not the water). “I sent pictures of manhole covers outside the 94th Precinct that have as much water as I do,” he said. Meanwhile, the offending birdbath is still there.

“That’s my mother’s birdbath,” he said. “That’s going to stay.”

One of the other four New Yorkers whose birdbaths ran afoul of Article 151 left the country before her case could be heard. Another has a hearing next month.

Last September, Mr. Pomares received a lesser fine, $300, from another hearing examiner who said the “penalty reflects prompt correction.” Mr. Pomares said that after he got the violation, he drilled holes in the $120 concrete birdbath to be sure that it drained and filled it with gravel. That should keep inspectors away, but also the robins and sparrows that used to splash in the bath.

“Mine was clean; I was a fanatic,” he said. “But I learned my lesson. If I had known about the law, I never would have bought it.”