It's like something out of an Alfred Hitchcock movie -- an enormous number of crows have been descending on an Ottawa neighbourhood each evening for the past several winters.

The gatherings at Ottawa's Coronation Park begin in the afternoon and swell as the sun goes down. More than 20,000 of the birds dot the sky and darken treetops in the park on some nights.

"The flock is growing and growing. You can hear them. It's this energy, it's almost effervescent," said bird-watcher Tony Beck.

According to recent research, crows may be one of the most intelligent species in the world.

Beck described them as social animals with long memories who, in this case, may be looking for safety in numbers. The larger the murder of crows, the more eyes and ears are available to watch for predators or even ward them off.

"I've watched them mob things like owls or hawks," Beck said.

"They have a variety of sounds and they appear to be talking to each other. They also seem rather playful -- I've watched them doing little somersaults and little bonding routines where they'll fly around each other," he said, adding that they tend to huddle together for warmth on cold nights.

However, some in the neighbourhood are less than thrilled about the eerie phenomenon, grumbling that the birds leave a big mess on the ground after they flock to a tree.

"It's a little weird," said one nearby resident.

An open letter written to a local city councillor in May called on police and fire officials to try to disrupt the crows' roosting pattern.

The letter, written by Ottawa resident Orrin Clayton, complained that the crows scared away songbirds in the park, that they might boost the risk of avian flu to those living in the area, and that they produce a cacophony of "intolerable" cawing.

While a murder of crows can be as small as a few birds, Chatham, Ont., is another community where much larger numbers have been known to congregate during the winter months.

Hundreds of thousands of the birds arrive each year and remain in the area roughly until March.

The city has taken a number of measures to combat the problem, including setting up a crow-control hotline and hiring a private company that flies hawks and falcons to scare them away.

"The Municipality does not condone the killing of birds," the city's website says, "we just wish to encourage them to move on."

With a report from CTV Ottawa's Alyshah Hasham