Raoul Schwing and colleagues published the finding in 2017. They broadcast a kea warble to wild birds in the Southern Alps and compared their behaviour with playbacks of other kea calls, a South Island robin song and a tone. Only the warble reliably triggered play: chasing, aerial tumbling, object play and solitary romping.
What makes the result unusual is that the warble did not merely recruit a partner. Birds already alone began playing by themselves, and birds already playing kept going longer. That pattern matches a call that induces a positive emotional state, comparable to laughter-like vocalisations described in rats and some primates, rather than an invitation directed at a specific individual.
The common overstatement is that kea laugh. The warble is not laughter, and no claim about humour is supported. What the study demonstrates is contagion of a play state through sound, which is a narrower and better-evidenced result.