By studying these birds, Schwing also realized that they have a “play call”—a warbling noise that they make while they’re playing. And the call isn’t just a statement. It’s also a trigger. It seems to induce playfulness in other keas within earshot; when they hear it, they also start playing with nearby peers. It has an infectious quality—just like human laughter. “When kids play and laugh, other kids want to play more,” says Schwing. “And this still exists in adults: That’s why American sitcoms have laugh tracks.”

Schwing first became interested in keas four years ago because they’re such unusual birds. They perch on an ancient branch of the parrot family tree: Around 56 million years of evolution separate them from other parrots like macaws or budgies. They’re very intelligent. In one study, a captive kea named Kermit successfully solved a logic puzzle in the most kea-like way possible—by rambunctiously attacking the problem until he found a solution. And they’re intensely curious, with a penchant for approaching tourists, stealing food, investigating bags, and wrecking cars. When I travelled to New Zealand, a kea tried to pry the rubber off my tires.

Unlike other birds, which shy away from new and unfamiliar objects, keas are drawn to novelty like moths to flame. Schwing once demonstrated this by asking one of his colleagues at the University of Vienna to shine a bicycle light at some captive ravens. Even though he had hand-raised those birds, they fled when the light came on. But when he did the same to captive keas, half the birds immediately flew over to investigate. This makes them very easy to study in the wild. “For most birds, you need hides and blinds,” says Schwing. “But if you see a kea flying in the distance, you can make as much noise as possible and wave things around, and they’ll make a beeline for you.”

Schwing started his work by building a library of kea calls. After two years and a lot of recording, he realized that the calls fell into seven categories, and that one of them was only ever made during play. To understand its role, Schwing and his colleagues took some speakers into the mountains and played one of five possible recordings: the kea play call, two other kea calls, the call of another local bird, or a simple standardized tone. They found that the play call—and only the play call—made keas more playful. For five minutes, they played more frequently and for longer bouts than the minutes immediately before. And once the recordings stopped, so did the play.

When the birds heard the call, some of them would join in play that was already underway. But many just started playing with the kea that was standing next to them, which was also not playing. And if there’s no other kea around, they’ll just play on their own, throwing rocks or performing aerobatics. Schwing sees human laughter as the closest analogy because both the call and laughter are positive behaviors that temporarily triggers something similar in other listeners.

Again, that might seem anthropomorphic, but Schwing’s going with it, partly because similar kinds of “positive emotional contagion” have been found in two other species—rats and chimps—and in both cases, the scientists involved “immediately called it laughter,” he says. “Hearing people laugh puts us in a better mood but also increases the humor that we perceive in certain things,” he says. “But of course, you’re not compelled to do somersaults, so the play call isn’t exactly like this.”