An argument can break out anywhere online, even in unsuspecting places.

So maybe it shouldn't be surprising that a picture of a duck sitting patiently in line for security at the Charleston airport would ruffle some feathers.

It started innocently enough: The Transportation Security Administration posted a photo of a mallard Thursday night, along with a few choice puns about the bird, which it described as a "service duck."

"The traveler assured us there was no 'fowl' play afoot and that this was simply her service duck," the federal agency wrote. "Our officers at Charleston (CHS) were overheard saying that this duck was pretty chill. Not lame at all ..."

At first, commenters reveled in the jokes and the idea of traveling with a duck. The good humor wouldn't last.

"There is no such thing as a 'service duck'. It would be considered an 'emotional support' animal," one Instagram user quickly retorted. "There's a huge difference, and people need to stop abusing the system."

They weren't wrong: Federal law requires airlines to allow emotional support animals on board to comfort passengers with conditions like anxiety. That's different than service animals, such as guide dogs, that perform specific tasks.

Within hours, the conversation took a turn.

Users said they thought the idea of an emotional support duck would diminish the role of service animals they described as "legitimate." Others defended the duck's legitimacy. And some wrote in about their phobias of birds and wondered who they could talk to at the airport if a duck was assigned to their flights. More than 400 comments volleyed back and forth.

And then, like any argument online, they got mad at each other for getting mad in the first place.

"This shouldn't tick you off," one commenter quipped. "Maybe you need a service duck."

In truth, emotional support animals like ducks aren't so uncommon, said TSA spokesman Mark Howell. A duck named Daniel went viral last year when it boarded a short flight from Charlotte to Asheville, capturing the internet's fascination as it looked wistfully at the clouds and posed for pictures with its "Certificate of First Flight" from American Airlines.

The duck, which passed through Charleston on June 21, wasn't the first unusual bird to clear security in the Lowcountry, either.

Howell said a support chicken came through Charleston International last year. Each bird was carried through a metal detector and swabbed for explosives, and their owners were asked to show a doctor's note verifying the bird was necessary.

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Elsewhere, monkeys have gone through security, along with all manner of cats and dogs.

"If the airline allows it, then we check the paperwork and the animal goes through screening, just like a dog," Howell said.

That's not to say airlines will just let any animal through.

Delta, Charleston's top air carrier, won't allow hedgehogs, ferrets and insects, among other animals, including waterfowl. Neither will JetBlue, the No. 3 local airline. Southwest, the second-largest player in the market, doesn't explicitly ban waterfowl, but it does prohibit "exotic animals."

Yet on June 21, when the mallard came to security, TSA agents let it through, Howell said. Apparently the passenger's airline was OK with letting it fly.

Weeks later, that's about all that's known about the duck. Unlike Daniel, the duck in Charlotte, no one's sure of the Charleston animal's name. Agents didn't ask, Howell said, but they did snap a picture.

And even if they had, he said, the TSA wouldn't name the duck publicly: As a policy, the agency won't disclose passengers' personal information.

Reach Thad Moore at 843-937-5703. Follow him on Twitter @thadmoore.