Patterns of feather mite (Acariformes: Astigmata) prevalence and load in a promiscuous bird during the breeding season

The effects of ectosymbiotic feather mites on avian host fitness and factors driving the variation in mite infestation levels, such as host mating and brood care system, are poorly understood. We investigated patterns of feather mite prevalence and abundance, and relationships between infestation and body condition in breeding Aquatic Warblers Acrocephalus paludicola, a promiscuous songbird with female-only parental care. In plumage, the dominant mite species was Trouessartia bifurcata, whose prevalence more than doubled during the breeding season, to reach 95% (95% confidence limits 71–99) during the second-brood period. Approximately 5% of birds were co-infested with Proctophyllodes cf. clavatus. While mite prevalence did not differ between sexes, mite abundance was significantly greater in Aquatic Warbler females than in males, and it increased between the first- and the second-brood period irrespective of sex. No significant relationship was found between mite prevalence or abundance and host condition expressed as mass scaled to size. However, in breeding females, this relationship could be confounded by the effects of nest-dwelling ectoparasites present in some nests but only sporadically if at all detected on adult birds; 2% of adult birds were heavily infested with the hematophagous mite Dermanyssus hirundinis (Mesostigmata). Our findings indicate that the mating system may affect symbiont variability between and within host species. The high prevalence of feather mites on adult birds at the late stage of the breeding season supports the hypothesis that promiscuous species have high infestation levels. Greater mite loads on Aquatic Warbler females than males reveal a different pattern from previously studied birds. However, we did not identify any consequences of mite infestation for the female body condition or current breeding success; hence, the apparently commensal relationship does not imply costs of female promiscuity.