Gifted songbirds such as the mockingbird and canary can vibrate each of their two membranes independently, producing two different, harmonically unrelated notes at the same time—a low-frequency sound on the left, a high-frequency sound on the right—and shifting the volume and frequency of each with such breathtaking speed as to produce some of the most acoustically complex and varied vocal sounds in nature. (This is quite extraordinary. When we talk, all of our pitch, all the harmonics of our vocalizations, move in the same direction.)
