Birds sing to advertise. Males do most singing, to announce that they hold a territory. The song warns other males to keep away, while enticing females to come closer. The song itself also identifies the species of the singer: it does not do to fraternise too closely with the wrong species. Because the song is usually precise it often enables the human listener to identify a bird that cannot be seen.
But listen carefully, for example to the Cape Robin. Although the basic tune, tone and volume are always the same, subtle differences exist between individuals. In fact every individual has a unique voice. Each robin recognises its neighbour’s voice exactly as we humans know a friend over the phone. This is very useful to a robin because a quick early morning burst of song tells everyone who is who, no need to spend unnecessary energy on old established relationships. On the other hand if a newcomer appears there will be much jousting in defining new boundaries.
The Chorister Robin has a more complex story. As well as its basic repertoire it learns new tunes – mostly calls of other species – with age; older robins know the most tunes. This is business, not fun. The robin with the greatest range of song has the highest social status. This makes the oldest males the most attractive. The system works because old robins don’t look any older than young ones. So an old widower gets first pick when choosing a new young wife. He will also get the wink from other young females looking for extra-marital affairs. What the little husseys want is the best possible genes for their next brood, while kidding their regular husbands to continue supplying family food.