The communicative value of complex singing in passerine birds

Birdsong is one of the most complex signals in the animal world, as it may consist of many different sounds grouped according to certain rules. Singing acts as a distant signal, indicating, e.g., the species and gender identity of the singer. However, territorial songbirds also use singing as an interactive social signal during territorial disputes, as well while interacting with female. In these contexts, males vary the type and timing of their songs to convey graded information about their motivational state, and those variations can play a role in communication. In this review, we considered how male songbirds vary their singing in territorial context. To study such variations, researchers usually simulated territorial intrusion by broadcasting conspecific singing in territories, including singing modified in a manner necessary for the researcher. For comparison, we considered briefly how singing vary in intersexual context. The author of the paper focuses on the role of singing complexity in communication. Therefore, not all known context-dependent changes in singing are considered, but only those related to “complexity”: the diversity of song/sound types and the transitional patterns of different song/sound types in the course of singing. Our review has shown that males change their singing when they detect environmental changes such as the appearance of a female or a competitor as follows: 1) song rate increases, 2) syllable rate increases, 3) song-type switching rate increases, 4) song-type diversity increases (i.e., the observed repertoire size), and 5) longer and more complex songs are predominantly used. In some species, the song bout organization may also change, but the data is still scarce. Typically, one or more, but not all the aforementioned acoustic behaviors have been found in a given song-bird species. All these behaviors (tactics) come down to a single strategy, namely: maximizing the acoustic diversity over a short period of time (e.g., several minutes), that is, increasing the number of different song and/or note types. The proximate causes of how the increased acoustic diversity work in the territorial competition context might lie in a sensory, or perceptual bias of the receiver. Namely, habituation should occur to repeated presentation of the same song type faster than to presentation of different song types. Therefore, by vocalizing more diversely, males more effectively influence the signal recipient’s behavior.