At a busy intersection near the Merrimack River in Lawrence, Craig Gibson set up his camera, peered through binoculars and got ready to witness a spectacle he's seen hundreds of times.

This recent winter day presented a scene worthy of a Hitchcock movie, as thousands of crows started streaming in overhead from several directions, growing to a swarm estimated to be about 14 to 15,000 birds. As they do every night during the winter, the birds all headed for a stand of trees near the intersection of Marston, Prospect and Canal streets.

There are other big crow roosts around, including in Springfield and Hartford, Conn., but Lawrence has the largest roost in New England, Gibson explained.

And in addition to being a source of fascination for local bird watchers like Gibson, the Lawrence crow roost is now a test case for government-funded research on new 3D imaging technology.

Gibson recently retired as the Catholic chaplain at Lawrence General Hospital just up the road from the crow roost. In that role, his job was often to provide comfort to people who were sick or dying. The crows became a way for him to regroup after a tough day.

“And so at the end of the day, this has been the most beautiful way to kinda buffer between the world of work of a hospital chaplain and then going home to have dinner with my wife or whatever the evening has in store for us,” he said.

The crows have grown into Gibson's passion. He created a blog about the roost, then a podcast. He's shared his photos and writing about the crows online with birding associations — that's where he heard about a research project funded by the U.S. Army Corps of Engineers. They were looking for a regular roost of thousands of birds to test out an advanced new imaging technology.

"I gave them details about this predictable repeat nightly roost phenomenon,” he said. “And they said ‘just the right size, exactly what we're looking for.’"

The military started studying the crows.

"The goal is to be able to measure the flight trajectory of each individual bird in a large flock,” said Andy Goodwin, a research engineer with the U.S. Army's Engineer Research and Development Center.

That kind of data could help answer some really interesting questions.

“For instance, why don't they bump into one another?” Goodwin said. “Or perhaps, you know, maybe — maybe they do. We just don't see it."

The military also wants to learn how to better avoid costly and dangerous bird strikes with their planes. They’ve partnered with Dale Fried, the CEO of a tech company called 3DEO.

"If we had one of these systems set up at Logan Airport, for example, and you were worried about drones — one or two or, heaven forbid, a whole swarm of drones coming in to do some damage to an aircraft, being able to track all those drones, figure out what they were doing, and devise a countermeasure, that’s really important,” Fried said.

To see whether they could do that, the Army awarded Fried a grant to test out a high tech imaging technology called Geiger-mode LIDAR. It’s like radar, but “radar uses radio waves to measure distance and we’re using light waves to measure distance,” Fried explained.

This kind of LIDAR system sends out pulses of laser light and does tens of millions of distance measurements every second on the small amount of light that comes back, Fried said.

Fried’s team rented out an office space on the top floor of an old Lawrence factory building overlooking the trees where the crows roost each night. A silver box the size of a mini-fridge was mounted in the window, with the device’s nickname, “Tweety,” printed on the side.

As the sun set, Fried and his team anxiously waited for the crows to show up, accompanied by the sounds of a ballet studio for kids that rented the room next door. Fried looked through binoculars as the crows began to arrive. They were hard to see from this distance. But the LIDAR system picked them up.

"There are times when there are these intermittent blooming flight bursts that are just dazzling to behold."
Craig Gibson

Fried pointed to a computer monitor with a 3D image of the stand of trees. The system detected each crow, and they appeared as dots as they moved around the scene.

"We’re kind of looking right over the tops of the trees,” he explained of the 3D image. “You can see the little clumps around there. And it's not just randomly placed. They're kind of moving in a systematic way. So that's very encouraging."

It’s that same collective movement that’s so fascinating to Craig Gibson.

"There are times when there are these intermittent blooming flight bursts that are just dazzling to behold," Gibson said as he watched the birds fly in. "And other times they kind of quietly work their way into the roost, settle down, and you don't see too many of those intermittent flight bursts ... Seven years out doing this and it's still, for me, it's as jaw dropping as the first time I was out here”.

Just then, thousands of crows that were sitting in the trees suddenly took off, filling the sky.

"Look at this!” Gibson exclaimed as they circled overhead. “Oh my goodness! How amazing! Oh wow!”

For a lot of bird watchers, the joy is in spotting something rare. Or appreciating colorful plumage. But not the crows. Gibson says there’s something different that makes this roost special.

"We're looking at this aggregation of flying wonders,” Gibson said. “They lift up, they go back down. And it's this dynamic action that's ever changing every night, that I think, for all of us, is just mesmerizing."

As the weather gets warmer, the nightly spectacle is coming to an end. The rest of the year these crows either migrate northeast or stay locally without roosting nightly.

But Gibson says he’ll be here to welcome them back again next winter.