Song complexity in male marsh warblers: does it reflect male quality?

Abstract

Male song is known to be important in the mate choice of birds, as different song features reflect different traits of the singer. The reason why song complexity, in this context, should reflect the quality of the performing individual is still under debate. Nevertheless, some studies show that song complexity may serve as an honest indi- cator of male quality. Evidence for but also against this comes from the genus of Acrocephalus warblers. Here, we examine the song complexity of male marsh warblers Acrocephalus palustris. They sing continuous songs com- posed of strophes of varying length containing different syllables, and are considered to be the most complex singers among Acrocephalus warblers. In terms of com- plexity, we refer here to the number of new syllables produced over time, and relate this complexity measure to female choice and male parameters reflecting parental and intrinsic quality. Males produced, on average, 11.6 ± 0.8 syllables per 6 s interval, and this number was highly repeatable over 120 s sampling period. Therefore, the structural complexity of male song can be accurately characterised from 6 s of singing behaviour, and eaves- dropping may allow females to assess complex songs more easily. We demonstrate a potential role of song complexity in female choice, as males singing more complex songs mated faster and their females produced bigger clutches. Furthermore, song complexity did not indicate the future paternal quality of the singer, and was not associated with male arrival time. The negative relationship between song complexity and strophe length may reflect different aspects of male quality.