They’re back.

If you’ve been downtown the past few weeks, you’ve seen the murders of crows hanging out during the day—but the true spectacle occurs on winter evenings, when thousands of crows convene for their nightly roost.

The birds begin by congregating at what ornithologists call a “staging area,” usually in the Park Blocks, for a “pre-roost.” The pre-roost is a sort of crow happy hour—an opportunity for crows from as far as 50 miles away to meet and socialize. To everyone who isn’t a crow, the scene is a cawing, flapping cacophony; then, without warning, they take off and fly to a roost site to sleep for the night.

It’s an awe-inspiring, vaguely apocalyptic phenomenon that’s poorly understood. But Gary Granger and Rebecca Provorse are aiming to change that.

Granger and Provorse are hobbyist birders turned amateur crow researchers, and their work, which involves tracking and counting Portland’s crows, is a perfect example of “public science” or “citizen science”—scientific research performed by members of the public as opposed to professional scientists.

I meet them on a chilly evening right before sunset in the South Park Blocks. We position ourselves in the center of the crows’ pre-roost activities—but not under any trees, in an attempt to not get pooped on. Every few minutes, our conversation is punctuated by a thunderous uproar of squawking as another group of birds swoops in to join the murder.

Provorse, a naturopathic endocrinologist, is full of energy and eager to share her knowledge. (Even when we’re talking, she keeps one eye on what the birds are up to.) The soft-spoken Granger, who works at Reed College, describes himself as an “urban naturalist” and has a wealth of avian knowledge. (He’s currently working his way through all the published academic research on crow populations.) Both have a contagious curiosity and love for the natural world.

They usually meet up when the crows do, staking out the pre-roost to see which direction the birds will fly in to get to their roosting spot.

“Sometimes we don’t get here in time for the pre-roost,” Provorse says. “So we just drive around until we find them.”

Once they’ve found the crows’ roost for the night, Granger and Provorse’s counting begins. Their notes and methods are meticulous, which is all the more impressive given how hard it is to track Portland’s crows. On a clear night, it’s nearly impossible to see the birds against the black sky, and when they roost, the birds are much quieter than when they’re cawing around during the day. In the dark, after they’ve roosted for the night, it’s easy to forget there might be thousands of silent crows above you.

Tree by tree and bird by bird, Granger and Provorse count, recording estimates of the number of crows for every block. Granger keeps a notebook with pictures and observations from each session.

“Right now, we’ve got more questions than we have answers,” Granger says. Purely out of curiosity, he and Provorse began their crow-roosting count in November 2017. Some of their questions: How many crows are roosting? Where are they roosting? And do crows move their roost during the night? For now, most of their research has only led to more questions.

To date they have performed 49 observations, primarily spotting between 5,000 and 7,000 crows roosting in downtown Portland, with the location of the roosts seemingly changing from day to day without any obvious reason. For their part, the crows aren’t inclined to offer answers: On one particularly cold, windy day, Granger and Provorse found them roosting on the waterfront, completely exposed to the elements.

Urban roosting only occurs during the winter months, and it’s not unique to Portland, taking place in cities across the US. There are almost as many theories on why crows choose cities as there are crows, but one of the main assumptions is that crows seek protection from the weather and the few degrees of extra warmth that an urban jungle provides. They also probably benefit from the safety of sleeping in large numbers, while city lights make it more difficult for predators to find and hunt them. But even though they use Portland’s skyscrapers as wind barriers, Granger and Provorse have found that crows only roost in barren trees, and never in Portland’s parks.

What is unique to Portland is the city’s imaginative responses to the crows. Thousands of crows roosting in downtown trees means thousands of crows pooping all over downtown, and the Portland Clean and Safe Program has led the initiative to handle this problem.

Their first attempt, in 2016, involved the purchase of a Zamboni-like street sweeper affectionately referred to as the “Poopmaster 6000.” But it quickly became apparent that scraping bird poop off the sidewalk was only a small part of the task: Droppings were still covering benches, bus shelters, stairs, and fountains.

So the group began brainstorming humane ways to tackle the sources of the problem, rather than just cleaning up the mess. Naturally, they landed on laser-trained hawks.

For the past three winters, Portland Clean and Safe has contracted with Integrated Avian Solutions, a bird abatement service (yes, apparently “bird abatement services” exist). Avian Solutions’ plan: Three nights a week, four falconers each fly a Harris’ hawk in an area that covers 72 blocks.

The goal of the project is for the hawks to scare the crows out of the project zone—just the sight of a hawk is enough to send crows skyward. Though Harris’ hawks have little interest in actually hunting crows, they’ve been trained to follow a laser beam; as their handler moves the beam from spot to spot, the hawks fly from perch to perch. Eventually, the handler will call the hawk back and give them a snack as a reward. This in itself is quite a sight: Since the hawks use gravity to slow themselves to a stop before perching, they dramatically speed toward the ground, then rise to settle on the handler’s arm.

This tactic of scaring off crows is called “hazing,” and it costs Portland Clean and Safe $60,000 a year. From the looks of it, it seems to be working: The Integrated Avian Solutions team now rarely encounters more than a handful of crows in the project area.

I meet falconer Adam Baz to shadow him on a shift. Wearing all black, he has a utility belt containing scraps of meat, a walkie talkie, and a neon vest that reads “CROW PATROL.” He’s all business, maintaining an unaffected tone while answering my questions. I’m unsure if this is an aspect of his rugged outdoorsy personality or because he has a super-fast falcon clutching his arm.

With the background noise of traffic and his fellow falconers chiming in on the walkie talkie, I can barely make out what Baz is saying for the first few minutes. Not helping matters is the fact that his hawk, Mars, is screaming to remind us he hasn’t been fed. On patrol nights, Mars has to work for his dinner.

According to Baz, the first few weeks of flying hawks in the city at night was a bit of a learning curve for both the birds and their handlers. Harris’ hawks normally hunt during the day, so during the first few flights, the hawks would “completely undershoot the perch they were trying to land on.” Meanwhile, the falconers—who usually fly their birds in open fields and forest spaces—had to learn to be vigilant of city hazards like cars and MAX trains.

Another unexpected element of the urban falconer’s job: interacting with the public. One can’t walk around downtown Portland with a Harris’ hawk without being noticed and asked questions. According to Baz, most people he interacts with see the crows as a nuisance and are grateful that he—and his hawk—are clearing them out.

Those negative feelings toward Portland’s crows run deeper than one might expect. While Portlanders gather every fall to watch Vaux’s swifts fly into a chimney in Northwest Portland, crows aren’t so lucky: Last January, both the Portland Oregon State Veterinary Diagnostic Lab and the Oregon Department of Fish and Wildlife confirmed that crows in Northeast Portland were being poisoned. The birds—which reportedly were “falling from the sky,” then having seizures on the ground—had ingested a neurotoxin called Avitrol. The culprit of these poisonings was never found, despite the fact the Portland Audubon Society offered a $1,000 reward for anyone coming forward with information.

It might seem like using laser-trained hawks to scare off crows is an overly antagonistic measure. But despite the fact that the Integrated Avian Solutions contractors are essentially terrorizing crows, they’re still bird-lovers. When Baz isn’t on crow patrol, he’s on the government payroll, working on bird conservation.

“We don’t want to get rid of crows,” he says. “They deserve to be here. They’re a native MBTA [Migratory Bird Treaty Act] protected species. They are incredibly intelligent, amazing birds. We are just trying to encourage them to adapt to our needs as well.”

Granger and Provorse, the citizen scientists, met once with the crow patrol to share information. After looking over old data, the two found that hazing dates coincided with major shifts in crows’ roosting sites that they previously thought were random—and that in the absence of hazing or other major disturbances, the crows roost in the same place every night. While Portland Clean and Safe views the hazing as a relatively humane solution, Granger, who recently shadowed the crow patrol, isn’t on board.

“We now know that the [crows’] movement was coincident with hazing by the hawks,” he says. “Current observational data indicates that, absent hazing or other external disturbances, the crows return to the same place each night. There is evidence that large nocturnal roosts persist for decades and longer.”

From Granger and Provorse’s perspective, the hazing is unwarranted: The crows’ droppings, they say, are barely noticeable, usually gone within a few days due to rain, and that hazing only shifts the location of the droppings to another part of the city. They also feel that the crows deserve to use the city to their advantage and that preventing them from roosting at their chosen sites may be detrimental to the health and survival of the crow population at large. But overall, they object to the hazing not “because we claim to have solid evidence that hazing the crows harms them,” Granger says, but “rather because there is no evidence at all in either direction.”

“It is now a goal to get the city, local Audubon folks, and others to see this as an amazing phenomenon and something to celebrate,” says Granger. He and Provorse have written their findings in a 16-page formal research paper that they’ve presented to the Audubon Society, Integrated Avian Solutions, and Portland Clean and Safe.

In the meantime, the city’s crows aren’t going anywhere. True, they probably won’t ever be as popular as the Vaux swifts. But watching crows—and the hawks who haze them—might be just the thing for Portland’s bird watchers who can’t make it through the winter without a fix.