Indigo

From Muppet Wiki

Indigo is a bluebird who is feeling blue. Big Bird figures he can cheer him up by shouting "Seventy-Six Trombones" in his face, until Bob suggests something more soothing. With a little help, Indigo begins to chirp along, but never speaks.

He is operated by two performers: one for the beak, and another who would use two hands for the inside of the wings.