Female blue tits sing frequently: a sex comparison of occurrence, context, and structure of song[Ultraviolet sexual dimorphism and assortative mating in blue tits]

In species with mutual mate choice, we should expect adaptive signaling in both sexes. However, the role of female sexual signals is generally understudied. A case in point is female birdsong that has received considerably less attention than male song. This holds even for well-studied species such as the blue tit (Cyanistes caeruleus), an important model in evolutionary ecology. Although there have been anecdotal reports of female song from three populations, there are no quantitative studies on female song in this species. Here, we report systematic sampling from a population of individually marked blue tits over 3 years, revealing that females sang frequently throughout the sampling period. Notably, daytime singing of females occurred in functionally similar contexts as in males (agonistic, solo song, and alarm contexts) but females had lower song output than males and were not observed singing dawn song, while males showed long singing displays at dawn before copulations take place. Female and male song overlapped substantially in acoustic structure (i.e., same song types or peak frequency) but females had smaller individual song-type repertoires, shorter trills, and lower vocal consistency. Differential selection pressures related to functional differences in male and female song might explain the observed variation in acoustic structure. With the first quantitative study of female song in such a well-studied species, we hope to stimulate further investigations into the functions of female singing, especially in the Northern temperate zones where female song may have been overlooked, not only in this but perhaps in other monomorphic species.