Why do birds make sound?

Many species of bird are more often heard than seen. Most birds have some kind of sound-making ability and they vocalise for a variety of reasons, including:

advertising territories

attracting a mate

deterring predators

making alarm calls

How do they do it?

Birds generate sounds from a structure called the 'syrinx', which is located at the junction of the two bronchi (air passages) in the respiratory tract (breathing organs). The syrinx is controlled by pairs of muscles. Generally, bird species with more muscle-pairs produce more complex calls. The songbirds, also know as passerines (of the order Passeriformes), possess between four and nine of these muscle pairs and make the longest and most elaborate calls.

The Ornithology.com website has more information about how and why birds make sounds.