when birds inhale, air is passed from the mouth, through the trachea, which forks into two bronchii connecting to the lungs. the primary vocal organ of birds is called the syrinx, which is located at the fork of the trachea, and is not present in mammals. as air passes through the respiratory tract, the syrinx and the membranes within vibrate to produce sound. birds are capable of producing continuous song during both inhalation and exhalation, and may sing continuously for several minutes. for example, the skylark ("alauda arvensis") is capable of producing non-stop song for up to one hour. some birds change their song characteristics during inhalation versus exhalation. the brewer's sparrow ("spizella breweri") alternates between rapid trilling during exhalation interspersed with lower-rate trills during short inhalations. the two halves of the syrinx connect to separate lungs, and can be controlled independently, allowing some birds to produce two separate notes simultaneously.