Why is birdsong so repetitive? Signal detection and the evolution of avian singing modes

Abstract

Signal repertoires, such as the song type repertoires of many songbirds, are thought to play a role in male mating success, with females preferring larger male repertoires over smaller ones. Yet, in many songbird species, males sing in a way that does not readily reveal their repertoire sizes by repeating each song type multiple times before switching to the next. Here I describe a potential explanation for such signal redundancy, based on the predictions of signal detection theory as it applies to intersexual communication during mate choice. According to this idea, a female’s response threshold to male mating signals (i.e., her ‘choosiness’) should select for male signal features that elicit favorable female responses (i.e., that are more ‘detectable’). Males can increase the detectability of their signals by producing them with higher redundancy, as well as by increasing their intensity and distinctiveness. Thus, in species with relatively high female response thresholds to males, such as taxa in which the sexes associate only briefly during breeding, males are expected to produce mate attraction signals that are especially stereotyped and repetitive. High signal stereotypy is also expected to be associated with features within signals that are relatively extravagant. Phylogenetic studies of a songbird group with a wide range of mating patterns, the oropendolas and caciques (family Icteridae), provide evidence consistent with these evolutionary predictions. Singing modes in this group have become more repetitive in some lineages along with the evolution of polygynous mating systems, even as various features within songs have become more extravagant.