Birdsong is one of nature's most beautiful sounds, but for the birds themselves it is serious business — a tool for survival and for raising the next generation.
To attract a mate
The biggest reason birds sing is to attract a mate. In most species it is the males who sing, and a strong, complex song advertises that a male is healthy, experienced and holds good territory — exactly what a female is looking for. This is why song is loudest and most frequent in spring, the height of the breeding season.
To defend territory
Song is also a way of saying this space is taken. By singing from prominent perches, a male warns rival males to keep out of his patch, often settling disputes without any actual fighting. A boundary held by voice alone saves everyone energy and risk.
Song versus calls
It helps to know that songs and calls are different. Songs are the longer, more musical sequences used mainly for mating and territory. Calls are shorter sounds used all year for things like raising the alarm, keeping a flock together, or keeping parents and chicks in touch. So a bird making noise is not always singing in the strict sense.