Bobby McFerrin’s voice was a uniquely impressive instrument Saturday night at the Warner Theatre. (All photos by Marlon Correa/TWP)

Actually, that’s not entirely true. McFerrin knew his role thoroughly, contributing to a series of complex and challenging vocal orchestrations drawn from his recent CD, “VOCAbuLarieS” — tunes that spanned a world of cultural influences. But during this a cappella performance, he sang with such effortlessness and invention that he often appeared to be improvising even when he wasn’t.

His voice was constantly shape-shifting: now a flute, now a bass, now a saxophone, now a — jeez, what was that, a didgeridoo? He didn’t sing Paul McCartney’s “Blackbird” so much as uncage it, and when the creature took wing, the wizardly Grammy-winner managed to create a fluttering Doppler effect in its wake.

Then, too, there were moments — many moments — when McFerrin simply tossed out the script. Early on, he was so moved by the choir’s rich and stirring harmonies that the sounds stopped him in his tracks — literally. The next thing you knew, he was channeling a street-corner serenader from the doo-wop era, lavishing praise on the choir in a Frankie Lymon-like croon.

Under the direction of McFerrin collaborator Roger Treece for this D.C. Jazz Festival performance, the choir deserved all the applause it received for navigating intricately devised vocal arrangements and contrapuntal passages and for fully charging the evening’s most inspirational song, “Messages.” Some two dozen singers strong, the group often generated syncopated or undulating waves of harmonies, then layered and contrasted them for dramatic effect.

With the choir behind him, stretching from wing to wing, McFerrin essentially performed in the round. One of the evening’s consistent pleasures was watching the young singers drop their jaws or smile in brilliant unison when McFerrin was engaged in some high-wire daring, either alone or with colleague David Worm. A singer and “vocal percussionist,” Worm matched wits with McFerrin during a freely improvised, bop-to-Brazil-and-beyond interlude.