Call- a brief sound of simple acoustic structure-a peep, cheep, squawk, chatter, etc. Not all birds sing and not all the sounds birds make are called songs. Calls are usually not as musical. Birds use calls to communicate many things to each other and between members of a flock or family.
Nearly half of the birds in the world do not sing. They are not, however, silent, nearly all birds use sounds to communicate. Most birds use vocalisations which are short and unmusical and cannot be termed as song. These sounds have considerable functionality and are generally labelled call-notes or calls to distinguish them from true songs.
Call notes can be divided into at least 10 different categories:
1) General alarm calls
2) Specialized alarm calls
3) Distress calls
4) Aggressive calls
5) Territorial defence calls
6) Flight calls
7) Nest calls
8) Flock calls
9) Feeding calls
10) Pleasure calls
Not all birds use these calls and many birds have more than one call for one category. To confuse matters even more, some birds seem to use very similar calls in different circumstances to mean different things. Most birds seem to have between 5 and 15 distinct calls. For example, alarm calls give information to other birds nearby. This applies not only to members of the same species but also to other species of birds. All these calls act as a simple language allowing birds to communicate with each other and with the rest of the world. In many environments the alarm calls of birds warn other animals of approaching danger.

Birds start using calls early in their lives, in some species even before they are hatched. Quail chicks use calls to communicate with each other and their mother from inside their eggs. They are able this way to synchronise their hatching so that they all emerge from the eggs within the space of a couple of hours. Pelican chicks tell their mum if they are too hot or cold from inside the eggs. Chicks also listen to their parents while inside the eggs. This way they come to recognise their parents even before emerging from the eggs.

 

Content Courtesy: www.earthlife.net/birds/song.html