Not all birds sing and not all the sounds birds make are called songs. Singing is limited to the order  Passeriformes, or perching birds. This means that 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 vocalizations, which are short and unmusical and cannot be termed as songs. These sounds have considerable functionality and are generally labeled call-notes or calls to distinguish them from true songs. Bird songs is a complex vocal signal that functions in a reproductive context to attract mates and defend territories. Male songbird vocalization can function to communicate with their mate.

House Wrens (Troglodytes aedon) are one type of songbird that utilizes vocalization for many different functions among the different stages of reproduction. 

One function of House Wren song in males is to attract a mate. When male House Wrens are trying to attract a mate, spontaneous songs are produced loudly in long bouts around unoccupied nest sites.

Vocalizations can also function as an ’all clear’ signal to females during nest-building, incubation, and raising young. By using song as an ’all clear signal,’ the female wren is able to leave or exit the nest without having to check for predators. House Wren song may also be used to ward off other males who threaten part of their territory. The vocalizations would help communicate to other wrens their ownership of a territory. Wrens have been found to increase singing performance when observing a moving intruder compared to a stationary one. References: 

Amrhein, V., and Lerch, S. 2010. Differential Effects of Moving Versus Stationary Territorial Intruders on Territory Defence in a Songbird. Journal Of Animal Ecology, 79: 82-87. G.F. Ball, in Encyclopedia of Neuroscience, 2009. Johnson, L. S. 1998. House Wren (Troglodytes aedon), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online. Johnson, L. S. and Kermott, L. H. 1991. The Functions of Song in Male House Wrens (Troglodytes aedon). Behaviour, 116: 190-209. Sackinger. D. 2013. Do Male House Wrens (Troglodytes aedon) Vary Their Singing Among Various Reproductive Stages? (Advisor J.K. Augustine) The Ohio State University.