There’s very little I can add to this, so I’ll send you over to an entry in Carl Zimmer’s blog, about a bird with curious wings.
This is probably no more complicated than evolving adaptive coloration or planting things on your body, but it’s a fascinating concept nonetheless. Birds developed an audible signal for communication that seems to work pretty well for them, but this is a species that developed an alternate method of doing so. To my untrained mind, this would seem to be a longer method to evolve than vocal cord variations, and one must wonder why this species departed from the norm (I believe it can still sing.) I mean, there’s a benefit to talking and eating at the same time, one I’ve wished I had, but I doubt that’s the key here. I can also speculate about the high frequency doing something for cleaning the feathers, but that’s little more than blind guessing. Still cool, though.