Bird Songs and Calls
The voice box of the bird is called the syrinx. It is located at the lower end of the trachea where the bronchi join. Sound is produced when the chest muscles compress a pair of air sacs that squeeze air from the lungs through the syrinx. This passage of air, in turn, causes the tympanic membranes, two pairs of elastic membranes, to vibrate, thus producing sound. Pitch and volume are regulated by varying the amounts of tension in the muscles supporting the syrinx. The complexity of the sound depends on the number of syrinx muscles acting on the tympanic membranes. Birds with only one or two pairs of syrinx muscles, such as ducks and geese, produce plain, simple sounds. Birds with five to nine pairs, such as songbirds, produce elaborate, complex sounds.
Birds produce two types of sounds: calls and songs. Calls are shrill and very short, usually of only four or five notes. Each species has a unique call or set of calls. They are used to signal the approach of a predator and to attract other birds to food. Songs are more complex. In most species they are sung only by the male in the spring, when they are triggered by the release of sex hormones. Songs are usually learned by the young through imitation. Bird songs are used to identify the location of the singer, to attract a mate, to establish a territory, and to warn others to stay away. Bird songs are variable even within a species. Birds living in different geographical areas develop dialects specific to their region.
The forebrain, an area in the forward part of the brain, controls song output. It expands during the mating season. It is generally larger in males than in females since males are usually the only ones to sing. The versatility and number of songs a bird can produce are proportional to the size of the forebrain.The green-backed heron has a loud and sudden call.