Birds sing both to defend their territories and attract mates. In most species, a male bird needs to “own” a territory before he can attract a female and breed successfully. A male claims his territory by singing in it. He usually uses shorter, simpler songs to defend it against rival males. When a male sings for females, his longer, more elaborate song shows he is fit and healthy—and would therefore make a good partner. Female birds also sing, to ward off other females from their territory and to signal to males that they are available for mating. In the tropics, females perform duets with their male partners to strengthen the bond between them. Each species of bird has its own variety of songs. And just as humans speak with different accents or dialects, some bird species develop these, too, often because populations have become separated from one another by, for example, mountains or stretches of desert.
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Birdsong

Birdsong is particularly vigorous in spring—the ideal time to attract a mate, because by the time offspring have hatched, the weather is warmer and there is plenty of food about. Native bird species, which stay all year round, sing in winter to defend their territory. But when the days start getting longer at the beginning of spring, the birds start singing for mates.

Why do many birds sing more intensely at dawn—what we call the dawn chorus—than they do at any other time of the day? No one knows for sure, but scientists think that many of the songs heard at dawn serve as warnings to other birds in defence of their territory and their mate. The air is more likely to be still in the morning, which means that the sound transmission is better. 

Birds use an organ in their throat called the syrinx to produce sounds. This is the equivalent of the larynx or voice box in humans. The syrinx contains membranes that vibrate and create sound waves when air from the lungs is breathed over them. The muscles of the syrinx control the sound the bird makes. Unlike our voice box, which is situated at the top of the trachea (windpipe), a bird's syrinx is positioned much lower down, at the junction of the two air tubes (bronchi) that connect with the lungs. Each side of the syrinx is independently controlled. This allows birds to produce a greater variety of sounds than we humans can—even two sounds at once.
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Though other birds can make short, simple calls—usually to sound the alarm, or just to keep in contact with other members of the flock—the songbirds (most of the perching bird group) have extra muscles in their voice boxes. This allows them to produce complex patterns of notes which we call songs. Young songbirds learn songs from their parents and other adults.