Symbiotic feather mites synchronize dispersal and population growth with host sociality and migratory disposition

Some symbiotic taxa may have evolved to track changes in the level and quality of food resources provided by the host to increase reproduction and dispersal. As a consequence, some ectosymbionts synchronize their reproduction and activity with particular stages of their host's living cycle. In this article we examined temporal patterns of variation in prevalence and abundance of feather mites living on pre‐migratory barn swallows Hirundo rustica. Feather mites in the lineages Pterolichoidea and Analgoidea are the most common arthropod ectosymbionts living at the expenses of feather oil. We investigated whether the seasonal variations in levels of several measures of physiological condition associated with host migration were related to changes in prevalence and abundance of mites. The results suggest that the variation in prevalence of feather mites, and thus probably the mode of acquisition and dispersal of these symbionts, is linked to an increase in host sociality before migration. Physiological dynamics of hosts after the breeding season point at two clearly identifiable periods: a post‐breeding period when physiological condition remains stationary or decreases, and a pre‐migratory period characterized by a rapid increase in several measures of physiological condition. Mite population dynamics were synchronized with migratory disposition during the period of highest host gregariousness. These synchronized processes occurred in both study years, although dynamics of migratory disposition and mite prevalence and abundance differ somewhat between years for adult and juvenile hosts. Mite population increase before host migration may be a response to a higher quantity of food provided by the host, namely oil from the urpoygial gland which is stimulated by hormones. Therefore, mites might have evolved to adjust their reproduction to the time when they have more chance of dispersal through horizontal transmission. In addition, body mass of juvenile and adult hosts were positively related with mite abundance in both years after allowing for several influencing factors. Body mass variation may reflect adequately fitness of host or their current physiological state, for instance, differences in the secretion of lipids on feathers or a more adequate microclimate to these symbionts.