Emily S. Rueb / New York Times News Service

NEW YORK — One swarm covered the side-view mirror of a Volvo station wagon in a lot by the Hudson River, trapping a family of three inside. Another humming cluster the size of a watermelon bent a tree branch in front of a Chase Bank in Manhattan, attracting a crowd of gasping onlookers.

And for several hours, thousands of bees carpeted a 2-foot tall red standpipe on the patio of a South Street Seaport restaurant, sending would-be outdoor diners elsewhere.

This spring in New York City, clumps of homeless bees have turned up at nearly double the rate of past years. A warm winter followed by an early spring, experts say, have created optimal breeding conditions for the insects. That may have caught some beekeepers off guard, especially the less experienced generation of keepers who have taken up the practice in recent years.

When Happy Miller, the Seaport restaurant manager, saw tourists flailing their arms in a cloud of airborne black specks late last month, he closed the glass door and quietly panicked.

“Oh my God, what do I do?” he thought before calling 311, security guards and local news outfits.

The television trucks, he said, were first to arrive. It took several hours before Officer Anthony Planakis, the New York City Police Department’s unofficial beekeeper-in-residence, arrived with a metal swarm box and a vacuum to collect the 17,500 or so homeless creatures.

Planakis, who has been responding to swarm calls since 1995, said this had been New York’s busiest year of swarming bees he had experienced. Since mid-March, he said, he has tended to 31 jobs in the five boroughs, more than twice the number he handled last season, which normally extends from mid-April through July.

“It’s been pretty hectic,” he said, adding that this week’s warmer temperatures could encourage more bees to take off.

This resurgence comes after several years of a puzzling decline in the honeybee population. Since 2006, beekeepers have reported that an average of 30 percent of their hives have vanished each year. The phenomenon — known as Colony Collapse Disorder — has prompted vigorous research to determine the cause but has so far yielded inconclusive and ambiguous results.

The swarms, while anxiety-provoking, have resulted in no major injuries. It can be difficult to trace a swarm to its source. Planakis said the bees he had collected were feral — meaning wild — but some beekeepers believe they were fleeing the poorly managed hives that have proliferated on rooftops, in backyards and on balconies since New York City lifted a decade-long ban on raising Apis mellifera — the common, nonaggressive honeybee — in March 2010.

Since then, 114 people have registered 182 hives with the city’s Department of Health and Mental Hygiene. Many others say they are reluctant to divulge the location of their hives for fear of retribution from landlords, neighbors and the city. Some estimate the actual number of hives may be as high as 400.

“There’s a stigma to a beekeeper whose hive swarms,” said Joe Langford, who watched from his kitchen window in Brooklyn on Mother’s Day as a 20-foot-high cloud of his bees blotted out the sun before landing on a nearby tree, causing a few muffled screams from below. Before he could think about where to get a ladder, they had vanished.

“I was awestruck and mortified at the same time,” he said. “For the rest of the day, I couldn’t stop looking up.”

While 311 will take complaints about bees, the New York City Beekeepers Association and NYCBeekeeping.org maintain swarm hotlines and may take the free bees.

This year’s unusually mild winter may have also allowed more bees to survive. Flowering plants and trees began blossoming several weeks early this year, causing colony populations to peak early. In the spring, a colony can explode tenfold to take in the season’s nectar. The early arrival of their spring broods could have caught beekeepers off guard; bees swarm when they are overcrowded, following a queen in search of a roomier hollow to call home.

“This year, all the rules were broken,” said Dennis vanEngelsdorp, a researcher at the University of Maryland, who co-led a national survey of managed honeybee colonies released this month.

It showed that about 21.9 percent of bee colonies nationwide died over the winter, a substantial drop from the 30 percent average losses reported in the previous five years.

When asked if this meant a rebound in the population, vanEngelsdorp said on Skype from Pretoria, South Africa, “It’s too early to tell.”

The state’s Agriculture Department estimates there are 60,000 to 70,000 colonies in New York. And the state’s 17 or so beekeeping clubs have generally doubled in size in the past five years, said Paul Cappy, the head apiculturist for the New York State Department of Agriculture’s Apiary Inspection Program.

A swarm is a perfectly natural phenomenon, Cappy said: “It’s good for the bee population, but not for the beekeepers.”