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“It might move slightly within the neighbourhood but it’s near the hospital.

“Over the years that roost has been up to 10,000 birds. It’s highly variable. It depends on how severe our winter is. There was a time when the American crow (that’s the formal name) was scarce in the winter, but everything has changed for various reasons — access to food and so on. So the numbers have increased.”

Crows dine well where people live. In the mornings they get an early start for cornfields in the Greenbelt, and the Trail Road landfill site. If they get desperate they may eat staghorn sumac berries.

They return to the roost near sunset.

“This goes on every year … In some American cities they have had the same roost for more than a century,” he said.

And if you think 10,000 is a lot of crows in one place, you haven’t been to Chatham. The southwestern Ontario city was plagued by a winter crow population that probably ran into the low millions in the 1990s, though it has dropped a long way from there. Some people say it’s now tens of thousands, others say it’s above 100,000.

Crows belong to a group called corvids, along with ravens and jays, and the corvids are all smart. When Chatham hired a hunter to shoot at them, crows figured out how high they needed to fly to stay safely above the pellets.

Seeing a big flock reminds Di Labio of the Hitchcock thriller The Birds, except that he likes crows.

“They’re very social creatures.”

But why would they stick together?