Agonistic song rate positively correlates with male breeding success and avian malaria infection in Acrocephalus paludicola (Aquatic Warbler), a promiscuous songbird with female-only parental care

The link between male song and reproductive success has been explored at length in socially monogamous birds, but results were inconsistent and under-represented socially non-monogamous species with uniparental care. Here, we evaluated whether metrics of male song in the aquatic warbler Acrocephalus paludicola, a promiscuous songbird with female-only care and large repertoires, are associated with fitness- and quality-related traits. We showed that the number of 9-day-old nestlings fathered and probability of avian malaria infection increased with the rate of agonistic song, produced in male-male interactions, and that male scaled mass index (proxy for energy reserves) positively correlated with the repertoire size. The male breeding success was not explained by repertoire size and song duty cycle. None of the song variables clearly explained the male return rate or change in the tarsus length (proxy for structural body size) and scaled mass index of the fathered chicks between days 2 and 9 post-hatch. No other relationships between the song characteristics and the inbreeding coefficient, scaled mass index, wing-length and Trypanosoma infection status of the males were supported, and Plasmodium infection was unrelated to the song duty cycle and repertoire size. We conclude that in male aquatic warblers the repertoire size could be a signal of early developmental stress or current body condition, and the agonistic song rate could be shaped by sexual selection and signal to females a genetic makeup that enhances survival upon infection by Plasmodium.

Methods
The data was collected from Aquatic Warblers in the Biebrza Valley, Poland, by ringing, blood-sampling, song-recording, nest-search and chick measurement. Body mass and tarsus length were taken from adult males and chicks. DNA was extracted from the blood samples and - using RAD-seq - inbreeding of the males was established and chick paternity was assigned to determine the breeding success of the males. Blood parasite infection was determined using PCR. Five-year return rate was established by resighting the colour-ringed males in consecutive years.