Birdwatchers classify a bird's vocal sounds as:
While this arbitrary distinction may be useful in identifying different species it is also useful to understand how birds learn to make these sounds and, indeed, how they make them.
Calls usually have a specific purpose. For example, a party of Long-tailed Tits or Goldcrests are usually constantly chattering to one another with contact calls and the Blackbird utters a rapid scolding alarm call when a predator appears.
Some species, like gulls and the Kingfisher, do not have songs, but most others and especially perching (passerine) song birds do. One of the best examples of bird song is the dawn chorus in the spring and summer.
The dawn chorus starts before sunrise with one or two birds singing, usually starting with the Blackbird, followed by the Song Thrush, Robin, Wren, Great Tit and Chaffinch. Gradually more birds of various species join in until there is a symphony of song by mid-morning.
The purpose of the dawn chorus is not really known, though there are several possible explanations:
Mostly it is the male bird that sings, though there are exceptions. For example, both male and female Robins sing during the winter when they are holding individual feeding territories.
Scientists have gathered evidence that suggests birds are both born with a basic ability and vocabulary to sing, but that they also copy or mimic their parent's calls and songs and those of other mature birds.
A juvenile bird will usually start to learn to sing with sub-song, or a subdued jumble of notes. Sub-song can be heard during autumn and winter: Robins and Blackbirds are good examples, and they can often be heard singing very quietly, as if to themselves.
Some species, such as Starlings, take their ability to mimic to the extreme and often mimic the sounds of other species and machines, such as wolf whistles and car alarms.
As well as developing their singing, there is also evidence that having an extensive repertoire of songs either impresses females or frightens the competing males.
A bird's vocal cord, or syrinx, is simpler than that found in humans. Instead of being located in the larynx (Adam's apple) at the top of the wind-pipe (trachea), it is located at the bottom, much closer to the lungs. The avian larynx at the top of the trachea does not serve any purpose in vocalisation and only prevents food and water from entering the lungs.
Sound is generated when air flowing through the narrow syringeal passage causes the tympanic membrane to vibrate in the same way a drum skin vibrates when struck. The muscles alter the tension of the tympanic membrane, like tightening a drum skin, and this alters the frequency or pitch of the notes. The position of the syrinx, at the top of the two bronchi, means that birds can sing two different notes simultaneously.
The reason a bird a small as the Wren sings so loudly is because the syrinx is a resonant cavity which maintains or amplifies the notes.