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Every day, the city of Chatham collects garbage from a different neighbourhood. And somehow, every day, the crows know exactly where to go.

They wait patiently on rooftops and tree branches as the sun rises. When the trash goes out, the carnage begins. Crows descend upon bulging plastic bags, disembowel them in search of food, and gleefully scatter the contents across front lawns.

Professional crow-chaser Ulrich Watermann just laughs and shakes his head as he watches the birds.

"People," he says, "are incredibly stupid."

Crows, on the other hand, are smart - smart enough to memorize garbage routes and a lot more. Humans, with our big brains and opposable thumbs, should theoretically be able to outwit them. Yet fat crows and messy lawns remain common sights, not just in this southern Ontario town, but in crow-plagued cities from Burnaby, B.C., to Charlottetown.

Humans have feared crows as evil omens and reviled them as urban nuisances, but only now are we starting to understand just how intelligent they are. Their cleverness in the lab and in the wild has made them scientific darlings and YouTube stars. But the cunning that fascinates researchers also stymies towns that are besieged by thousands of noisy, messy crows every winter.

The battle between birds and humans has spawned a cottage industry of professionals such as Mr. Watermann, whose bag of tricks includes pyrotechnics, lasers, recorded distress calls, birds of prey and wooden noisemakers.

He constantly changes his mode of attack, hoping to annoy the crows so much that they leave.

If the birds flee to a neighbouring town, he'll be happy to help them too - for a fee. Chatham pays Mr. Watermann about $32,000 a year.

Lean and dressed in black, with glittering blue eyes and a weathered face, Mr. Watermann could pass for an overgrown crow himself. Like most people who study crows, he's grown to admire them.

When hunters fired shotguns at the Chatham crows, he says, the birds figured out exactly how high to fly to escape the pellet range. When city workers made nightly rounds to disturb them, the crows learned that the workers clocked out at 11 p.m., and simply waited until 11:01 to head into town for the night.

"They are highly intelligent as a single bird. Then they come by the hundreds of thousands, and they all learn from each other. If that one makes a mistake," he says, pointing to a crow regarding him suspiciously from a wire, "the next one doesn't make it."

In labs, crows have demonstrated that they can not only use tools, they can make tools - bending a straight wire into a hook to pick up a basket of food, for instance. They will also use one tool to fetch another tool for a specific task, a skill known as "meta tool use." With special "crow cams," researchers have observed similar feats in the wild. Experts say crows rival primates in intelligence.

Smart crows also make great video. One BBC clip on YouTube shows crows dropping nuts onto a busy road to be run over and cracked open. Then they wait for the crosswalk signal to fly down and retrieve their treats in safety.

One YouTube commenter wrote in response: "If they had hands like us humens [sic] they would take over the world and enslave us!"

Residents of towns where they roost may be forgiven for thinking the crow revolution has already arrived. Mr. Watermann believes the Chatham roost once numbered in the millions. (Current estimates range from tens of thousands to 200,000.)

At the peak of their infestation, crows pooped on houses, churches, businesses, cars and sidewalks. They wriggled into finished engine blocks at the local truck-assembly plant, leaving brand-new engines coated in guano and feathers. Residents carried umbrellas on sunny days.

"I hate crows," says Scott Heuvelmans, owner of a Chevrolet-Cadillac dealership in Chatham, where they used to wash every car on the lot daily. Now they're down to weekly washes, but he'd still be happy to see the last of the crows.

"I wish somebody would just go out and shoot them," Mr. Heuvelmans says.

The piercing cries and blackened skies as crows descend en masse inevitably trigger comparisons to Alfred Hitchcock's The Birds.

"I hate that movie," says Kevin McGowan, a Cornell University ornithologist and crow expert. A winter roost - or to use the poetic term, a murder of crows - "is not spooky, it's not unnatural; it's the most natural thing in the world," Dr. McGowan insists. "They've been doing this for millions of years."

Indeed, crows have always flocked together on winter nights for warmth and protection from predators - usually a few sentinels keep watch while the rest sleep or socialize. Some roosts date back centuries.

But it wasn't until the 1980s that people started noticing crows roosting in North American cities, Dr. McGowan says. He's got theories but no answers as to why. Crows like the heat of cities and the streetlights - they can't see well at night, so the light gives them an edge over owls and other predators. The garbage in town is a nice perk, Dr. McGowan says, but he believes most crows do their serious eating in fields.

Chatham in particular attracts crows in winter for the same reason Florida attracts snowbirds: location. The city of about 45,000 is sandwiched between lakes Huron and Erie, at the convergence of two major migration routes. The river running through the centre of town provides warmth, and fields of cash crops nearby offer plentiful food. You'd be hard-pressed to design a better crow haven. Except, of course, for the crow-disturber with his pyrotechnics and trained hawks.

Mr. Watermann realizes his methods may seem uncouth, especially when one of his hawks kills a crow instead of just scaring off the flock. But he figures a few dead crows help save thousands that would otherwise be shot or poisoned.

Like Chatham, other cities are turning away from killing crows. That's partly because lethal methods often don't work - Chatham was overrun by so many crows that even the most trigger-happy hunters didn't dent their numbers, and poisoning may kill other wildlife indiscriminately. But it's also because the corvus fan club is growing.

"It's not just their intelligence, it's their attitude," says Laurie Ulrich Fuller, a graphic designer in Lancaster, Penn., explaining why the crow became her favourite animal after 60,000 of the birds invaded her town two years ago. "They saunter when they walk; they don't pip along like a little bird."

Lancaster, 588 kilometres southwest of Chatham as the you-know-what flies, uses harassment tactics similar to Mr. Watermann's to disperse crows.

As a founding member of the Lancaster Crow Coalition, Ms. Fuller struggles daily to change public opinion. She shows the BBC video to local groups, which she thinks opens some minds.

But even bird watchers can be shallow, disdaining wisdom in favour of beauty.

"They're not pretty birds. Crows don't even get the attention of bird lovers. They'll go ape over a goldfinch, but not a crow," she sighs. "Which is a shame because they're far more interesting and far more intelligent."

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