The duckling showed up at their house after Cannarelli’s husband saw a Facebook post by an elementary-school teacher looking to rehome two ducklings. Her husband said he planned to keep them at their house just for the weekend before bringing them to the local garden centre he runs. One of the two ducklings died on the first night.

“We don’t know why,” Cannarelli said, explaining that they believe the bird was ill when it arrived at their home.

The surviving duckling, an American Pekin they named Louie, never made it to the garden centre. They kept him because Cannarelli suspected the tiny duck was bringing Barley back out of her shell.

“Maybe she was lonely, I’m not really sure,” Cannarelli said, adding that their vet said behavioural changes can happen when dogs start epilepsy medication.

Within a few weeks, Barley and Louie were inseparable. Every morning, Barley would go to Louie’s outdoor pen, which has a wind-protected shed and a small pool.

“You don’t expect that friendship, and it’s nice to see,” Cannarelli said.

Pekin ducks are domesticated, cannot fly and don’t exist in the wild. They can live up to 12 years.

“The duck is constantly following around the dog,” said Cannarelli, who has two children, ages 9 and 11. “They roll around together all day long.”

They occasionally wrestle – though always gently, Cannarelli said.

“Every now and then, she’ll get the zoomies and romp around and play with the duck as if he’s a puppy.”

They also spar over food. Louie’s favourite foods are tomatoes, peas and cucumbers, which Barley enjoys, too. They often snag bites from each other’s dishes, and Louie prefers eating out of a dog bowl to eating out of a duck feeder.

“The duck has no idea he’s a duck. He thinks he’s a dog,” Cannarelli said.

That impression isn’t just a joke, said Stanley Coren, professor emeritus of psychology at the University of British Columbia and the author of How to Speak Dog.

“If you’re getting these heterogeneous species living together,” Coren said, “they will pick up on the behaviours of the others who they’re living with.”

Golden retrievers, Coren added, are among the breeds most likely to form this kind of bond with another species.

“If you presented a duck to a terrier, the duck is gone,” Coren said. “Retrievers are much more interested in retrieving than they are in hunting.”

In this case, the timing mattered, too. Although dogs are predators and ducks are prey, young mammals give off pheromones that can sometimes trigger a protective instinct in animals around them – especially females, according to Coren.

“That means the initial contact which the dog makes with the duck is going to have this protective mothering kind of a halo,” he said, noting that he doubts the bond would have formed the same way if Louie was an adult duck when he met Barley. “Over time, that can turn into a friendship.”

Coren said he is not surprised Barley perked up after Louie appeared.

“Dogs are highly sociable and so are ducks, so they feel more comfortable having somebody familiar and nonthreatening nearby,” he said. “There really is an emotional bond, and it can be cross-species.”

Shortly after Louie arrived at their home, Cannarelli started an Instagram account to chronicle their unusual relationship. She had been sending her colleagues photos and videos when they suggested she make an account.

“I started it as a joke for the office girls,” Cannarelli said.

For two years, the account had a relatively small following, but in the past few months, it suddenly took off. Some of her videos have drawn millions of views.

“I’m completely blown away,” said Cannarelli, who refers to Louie as Barley’s “emotional support duck”.

People online are charmed by their dynamic.

“This beautiful story should be a book for kids,” someone commented on a recent post.

Cannarelli said having a pet duck is not for everyone.

“They’re very messy, they’re a lot of work,” she said. “You really have to love them and take care of them and keep them clean.”

Still, she said, Louie has become a fixture in their family.

“We all got attached to him … It’s not something I thought was ever going to be in my future, but now that he’s here, I seriously love him,” Cannarelli said. “He’s even on our Christmas card.”

She said she plans to continue sharing Barley and Louie’s friendship.

“I want to give people something to smile about.”