This is a true story about pigeons wearing cowboy hats. It happened in Vegas.

The day started out typically, according to Bobby Lee, our story’s main witness. On the afternoon of Dec. 5, he was on his way to the grocery store when he noticed something red outside his car window.

Pigeons were milling around a parking lot near a dumpster, pecking the ground. Ordinary behavior for the birds, but for one detail: Two of them were wearing miniature cowboy hats, one red and one gray.

Baffled, he grabbed his phone. The “birds have hats on, bro!” Mr. Lee exclaimed in a 12-second video that he later posted on Facebook.

Mr. Lee, 26, said he threw some Doritos out the window for the birds, but it scared them off. They didn’t go far; one settled on a ledge. On the internet, however, the cowboy-hat-wearing pigeons (watch out, pun incoming) took wing.

“It got a lot of attention fast,” Mr. Lee said. “The day after, I had a lot of news people texting me and people trying to buy my video.”

Storyful, which verifies and publishes material from social media, reached out to Mr. Lee and distributed the video. Local news organizations picked it up.

KTNV asked the obvious, most pressing question: “Who did this?”

By Tuesday morning, the video had been viewed, shared and commented on hundreds of thousands of times.

Mr. Lee said he had no idea who had put the hats on the birds, though he noted one coincidence: The Wrangler National Finals Rodeo was in town. But the Professional Rodeo Cowboys Association, which organizes the event, “had nothing to do with the pigeons wearing cowboy hats,” according to Scott Kaniewski, the editor of ProRodeo Sports News.

The mystery deepened. Animal welfare agencies contacted Mr. Lee, including Lofty Hopes, a bird rescue organization, which told him to stay on the lookout for the birds. Mariah Hillman, a founder of Lofty Hopes, said she had a second report of a sighting on Monday in the same area, as well as an unverified report of pigeons wearing hats in Reno.

The Las Vegas Metropolitan Police Department said it “does not appear to be a police matter at this time.” A Clark County spokeswoman, Stacey Welling, had no immediate comment.

Online, people expressed worries about the pigeons’ welfare. How were the hats staying on their heads — was it glue or some other adhesive? And were they cowboy hats, or top hats? (The New York Times’s fashion critic, Vanessa Friedman, was generous enough to make a determination: “cowboy hats.”)

Charles Walcott, a Cornell University ornithologist who has been studying the common pigeon for 30 years, agreed to do a deep dive into the work of the mystery bird milliner and answered some questions after being sent the video on Tuesday. Do the pigeons look O.K.? What shape are their skulls? How would a person go about putting a hat on a pigeon?

His first takeaways: “I enjoyed the video. I just thought those pigeons with hats were cute.”

After watching the video, Dr. Walcott said he was not too worried. The pigeons strutted around and pecked for food or pebbles for their gizzards, apparently unperturbed by their headwear. Though they looked mature, he could not determine the age or gender of the birds.

“I think the thing that I would emphasize is I can’t see that it is causing any great harm to the pigeons,” he said. The hats are “certainly light enough,” he added. “They look like happy pigeons to me. It is hard to know, of course, because they will not talk to us.”

But pigeons are not conducive to hats. The shape of their skulls is “fairly flattish but rounded,” Dr. Walcott said, so a hat must be fitted on the surface of the bird’s head. Whoever did that was apparently careful not to obscure the eyes, as far as Dr. Walcott could tell. “They certainly are not impeding the pigeon’s vision of things that it is pecking at on the ground,” he said.

But the hats “might get in the way of seeing a hawk coming down from on top.”

When Dr. Walcott was studying pigeons at Harvard in the 1960s, he and other scientists affixed light copper coils to the birds’ heads to create magnetic fields that would help the researchers understand how the animals use signals to home. The coils were fastened with a veterinary cement, similar to chewing gum, that could be peeled off the birds’ feathers.

“I would like to think that is what these folks used,” Dr. Walcott said. “But I bet it is not.”

Pigeons are adaptable, he said, and they love big cities, with all the windowsills available for nesting. The hat-wearing pigeons in Las Vegas could be captured with simple wire cage traps, propped open with a stick and a rope attached to pull when the pigeon goes inside to eat. Or the hats might eventually fall off.

Ms. Hillman of Lofty Hopes, the bird rescue organization in Las Vegas, said in an interview that for five days, she and her team had been canvassing the east side of the city, handing out business cards and telling residents to call if they see the birds. Another person reported seeing the pigeons on Monday, she said, and they apparently still had their hats on.

“The hats can molt off, but that will take time,” she said.

At this point, she and her staff are just guessing who fitted the pigeons with their accessories.

“Humans basically just need to keep their hands off animals,” Ms. Hillman said. “It is their life. They have the right to live free from harm.”