The sounds that birds produce can be categorized into two distinct classes: calls and songs. A call is usually a short and simple vocalization that signals flight or danger and is produced throughout the year (Catchpole & Slater 2008). A song tends to be a long and complex vocalization produced during a breeding season. Songs are organized into several phrases (or motifs) which consist of series of syllables (Figure 1). Syllables, in turn, are made up of a collection of single notes (or elements). Each individual bird has its own song repertoire, which consists of different versions of a song, called a song type. There is large variation in repertoire size between species. In about a third of all songbird species, birds have only a single song type in their repertoire while in about 20% of all species, the repertoire consists of more than five songs (MacDougall-Shackleton 1997). In some cases, such as brown thrashers (Toxostoma rufum), song types can exceed 2,000!



Figure 1: Schematic sound spectrogram of a white-crowned sparrow (Zonotrichia leucophrys) song Arrows indicate phrase (or motif) and numbers indicate syllables which are made up of notes (or elements), the simplest unit of song. © 2010 All rights reserved.

Species also vary in when they can learn new songs. Species such as white-crowned sparrows (Zonotrichia leucophrys) have a short period of a few months after hatching when they can learn songs (called close-ended learners) (Figure 2). In contrast, open-ended learners like European starlings (Sturnus vulgaris) and canaries (Serinus canaria) can learn and add new songs to their repertoire throughout their lives, usually on a seasonal basis. Which sex sings tends to depend on who competes to breed. In many species, only males sing; in others, both males and females sing equally in duets.