Plumicolous feather mites are a little-known but diverse and abundant group of symbionts of birds. The nature of the feather mite versus bird relationship is controversial, with reports ranging from suggestions of parasitism to evidence supporting commensalism or even mutualism. In addition, little is known as to why enormous inter-individual variation in mite loads is observed within bird species. In this study we show that migrant European robins Erithacus rubecula differ in their load of Proctophyllodes rubeculinus and Trouessartia c.f. rubecula by a factor of 2 to 3 depending on the winter habitat that they occupy. Such variation cannot be explained as being due to robins from different geographic origins wintering in different habitats, as wing shape and stable hydrogen isotope ratios (inferring breeding origin) in feathers of those robins did not explain variation in mite loads. Furthermore, there were no age or sex effects on the number of mites, but abundance increased as winter progressed. Robins wintering in the best putative habitat had higher mite numbers and the fact that there was no correlation between mite infestation levels and either muscle or fat scores indicates minimal costs to the host and hints at a commensal or mutualistic relationship. More research is needed to better understand the factors underlying differences between habitats. Meanwhile, habitat must be taken into consideration in future studies of the relationships between feather mites and birds.
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