Song features correlate with male reproductive success and avian malaria infection in a promiscuous songbird with female-only parental care

Abstract The link between male song and reproductive success has been explored at length in socially monogamous birds. However, results were inconsistent and under-represented socially non-monogamous species with uniparental care. Here, using as a model the Aquatic Warbler Acrocephalus paludicola, a songbird with no sexual dimorphism, female-only care, promiscuity and elaborate song, we explored whether the repertoire size, percent time singing (song duty) and rate of A-songs, used in male aggressive interactions, are associated with male fitness- and quality-related traits. We found that the seasonal breeding success (number of 9-day-old nestlings fathered) increased with repertoire size but was unassociated with song duty and A-song rate, and none of the song variables explained return rate. Hence, greater repertoire sizes could improve chances for copulation or nestling survival. Repertoire size and A-song rate were associated positively with growth of chick tarsus (proxy for structural body size) and negatively with change in chick scaled mass index, SMI (proxy for energy reserves), between days 2 and 9 post-hatch. Hence, nestlings of males with greater repertoires and A-song rate could fledge faster. The male quality-related traits: inbreeding coefficient, SMI, wing-length and trypanosome infection were unassociated with the song characteristics. However, while plasmodium infection was unrelated to song duty and repertoire size, it positively correlated with A-song rate. This could arise through better survival of higher-quality infected individuals, or modulation by testosterone. We conclude that in male Aquatic Warblers repertoire size and agonistic song rate predict some of their fitness- and quality-related traits, and could be adaptive.