Birds can sing two different notes at the same time using a specialized organ called the syrinx.

The syrinx is a unique vocal organ found almost exclusively in birds, located at the base of the trachea where it branches into the two primary bronchi. This anatomical positioning is fundamentally different from the human larynx, which sits much higher in the throat. Because the syrinx is situated at this junction, birds can pass air through both bronchial tubes at once, vibrating separate membranes on each side.Research conducted by scientists like Peter Tyack and others at institutions such as the Cornell Lab of Ornithology has shown that some species, like the Wood Thrush, can produce rising and falling notes simultaneously. This 'two-voice' phenomenon occurs because the muscles surrounding the syrinx can tighten or loosen the membranes on the left and right sides independently. This allows for the creation of internal harmonies and complex trills that are physically impossible for mammals to replicate.The efficiency of the syrinx is remarkable, as it uses nearly 100% of the air passing through it to produce sound, whereas a human larynx is far less efficient. Studies using high-speed imaging have revealed that songbirds can switch between sides in milliseconds or use both sides to create a single, richer tone. This evolutionary adaptation is primary used for mate attraction and territory defense, where complex songs signal the physical fitness and cognitive health of the bird.