They perch on branches and lamp posts, watching the people below them walking by.
When they spot something that catches their eye, like a plastic bag that could potentially carry food, they strike.
Flapping their big black wings, the crows swoop down on their targets, peck at their victims and startle them. This has been happening over the past few days, just outside Hougang MRT station. Men, women, children - no one is exempt.
About half a dozen crows began their attacks on Tuesday evening, reported Chinese evening daily Lianhe Wanbao.
When The New Paper visited the area on Wednesday at about 7pm, we spotted at least five big adult crows swooping down on unsuspecting people.
We followed some of the birds and were led to a small tree just outside Hougang Mall on Hougang Avenue 10, opposite Block 425. A few crows nestled in the tree while the rest perched on top of lamp posts.
We noticed that they would particularly strike at those carrying plastic bags walking near their tree.
In half an hour, seven people were attacked. All the victims did not see the crows flying down on them.
By the time they felt the crow's beak and the flap of its wings, the offending bird would have flown off.
While some people shrieked in shock, most were left puzzled and looking around before they hurried away.
A woman in her 20s was walking past the tree with a plastic bag of groceries in each hand when a crow flew down and pecked her head.
She started, spun around and immediately swung her plastic bag at the bird.
She said: "I'm okay. I was just shocked, that's all. Perhaps the crow was hungry."
Ms Aini Abdullah, 41, who works at a booth at the carpark near Hougang Mall, said the crows appeared in numbers on Monday.
"One afternoon when I was in the ticketing booth, I heard loud noises from the roof. When I came out of the booth to check, I saw many crows perched on top, pecking away at my roof. It shocked me," she said.
A biodiversity expert told TNP that the attacks could have stemmed from people feeding them.
Assistant Professor Frank Rheindt from the National University of Singapore's Department of Biological Sciences said the attacks were likely carried out by house crows, or Corvus splendens.
"What they're doing could be misinterpreted as them attacking people, but they may actually just be wanting to be fed.
"Like monkeys, they are incredibly smart and they are able to recognise plastic bags, which they think will have food for them," he said.