Biotic and abiotic factors affect parasite loads: feather mite load influenced by salt exposure, age and reproductive stage in the Seychelles warbler, Acrocephalus sechellensis

Many factors may affect symbiont distributions within host populations. Intrinsic factors, such as genotype, body condition and age may account for variations in symbiont loads between individuals . However, abiotic factors may also contribute to variations. We investigated correlates of variation in the number of feather mites, Trouessartia sp. (Trouessartiidae), per individual in the Seychelles Warbler Acrocephalus sechellensis on Cousin Island. Warblers from territories exposed to high levels of salt spray had lower feather mite loads than warblers from territories unaffected by salt spray, and juveniles had higher mite loads than adults. When the effects of salt spray were controlled for statistically, incubating birds had lower mite loads than birds in other stages of reproduction. Thus, an extrinsic and two intrinsic factors contribute to predicting feather mite loads.