Before Owen Williams and his friends got cozy on the Cambridge resident’s shady outdoor patio last month, he gave them a warning.
“I said, ‘Just so you know, you may get an acorn dropped on your head,’ ” the 39-year-old software engineer said.
Nobody got pegged on the noggin that evening, but the group still had to evacuate — a decision prompted by squirrels foraging high above and sending acorns down like tiny bombs.
“They were just dropping onto us. They would be pinging onto the grill and the chairs,” Williams said. “It was enough of a carpet-bombing that we had to basically go inside.”
It’s autumn, and if you’ve found yourself anywhere near an oak tree, you’ve probably noticed that this acorn season has produced a bit of a bumper crop. They pop-pop-pop in the road as they’re crushed by cars. On the sidewalk, pedestrians tiptoe around them. They’re the bane of the bike lane.
The region appears to be experiencing a “mast year,” experts say — a natural phenomenon in the boom-and-bust life cycle of oak trees. Certain conditions, including the weather, lead the trees to produce a higher volume of fruit every few years.
“There’s definitely tons more around,” said Marjorie Rines, a naturalist with Mass Audubon. “It’s been coming down pretty hard and fast.”
A few people in recent days have lodged requests about acorns with the city through its BOS:311 constituency service, asking for a street sweeper to pass through their respective neighborhoods.
“Constituent reports many acorns in the street making it dangerous to walk down,” one report said.
Craig Leite of Stoughton was among an unknown number of acorn victims.
As the 26-year-old was walking on a paved trail in Needham recently, acorns were “falling from overhead like tiny meteors.”
While looking at his phone and worrying about what was coming from above, he miscalculated the abundance of acorns on the path ahead.
“My foot hit a patch of them, and they act like marbles on this paved ground,” he said. “I lost my footing, I’m flailing around, and I ended up falling.”
There was slight bruising, but it was mostly his ego, he said.
While driving through Chestnut Hill to drop her kids off at school, Richelle Smith, 37, of Roxbury started hearing strange “dings” on her vehicle, she said. The first time, she thought it was kids being kids, maybe tossing objects at her car. Later, one hit the windshield and she figured it out.
“I’m saying, ‘Dang! What is going on?’ ” Smith said. “They hit hard. They’re falling from a very healthy, strong, vibrant tree. Chestnut Hill is definitely popping with acorns — or acorns are popping, whichever you want to say.”
Mark S. Ashton, professor of forest ecology at Yale School of Forestry and Environmental Studies, said, “It is a mast year but not a big one.” The proliferation of acorns is most noticeable in urban and suburban environments, where oak trees can make up much of the canopy, he said.
“All of these trees are open-grown trees,” Ashton said. “They aren’t competing with other trees for sunlight,” giving them more resources to allocate to flowering and fruiting.
“I can usually get a pretty good handle on how the public is perceiving nature by the kind of phone calls I get,” she said. “I get a lot of calls when there’s something that concerns people . . . but I have not gotten any calls about people complaining about acorns.”
She also noted that the large quantity is a hardly a bad thing. Many critters feed on the acorns, including squirrels, chipmunks, and some species of birds.
“Animals basically have three things they have to keep in mind: reproduction, avoiding predators, and eating,” Rines said. “And feeding is critical this time of year, so the acorns are great for that.”
Meteorologist and horticulturist David Epstein,who writes a weather column for the Globe, agreed.
“It’s not bad — it’s nature’s way of supporting critters with more acorns on a rotating basis, so it’s a way to regulate nature,” he said of the boom and bust cycles. “It might be a pain in the butt for a car or bike, but it’s a necessary thing.”
Michael Dosmann, keeper of the living collections at the Arnold Arboretum of Harvard University, said more acorns doesn’t just mean animals will be “very happy” — it also has implications for future forest tree growth.
“When you’ve got a heavy mast year, after that you’re going to have what they call more seedling recruitment — there’s just more oak seedlings, which is a good thing,” he said. “Those acorns germinate, and they become the seedlings, and they replace the forest.”
Williams, the Cambridge resident who had to readjust evening plans to account for the squirrel activity, said he understands the benefits of a plentiful acorn season — and not just because of how it affects nature.
On a recent night, he overheard what sounded like a conversation between a father and his young daughter outside.
“The dad stopped and took the opportunity to teach his daughter how to whistle with an acorn cap, just outside my window,” Williams said. “It was pretty cute.”