Host–symbiont relationship and abundance of feather mites in relation to age and body condition of the barn swallow (Hirundo rustica): an experimental study

We analyzed the host–symbiont relationship and factors determining the abundance of feather mites among individual barn swallows (Hirundo rustica L., 1758) in two different host populations during the breeding season and postbreeding period. By experimentally removing the feather mites from the flight feathers of the birds with an insecticide, we showed that these symbiotic organisms have neither harmful nor beneficial effects on the fitness of the host, supporting the view that mites are commensals. This was indicated by the lack of any difference in the change in wing-feather length, tail-feather length, fluctuating asymmetry in tail-feather length, breeding performance, and survival of the birds between the fumigated and control groups 1 year after the experiment. During the postbreeding period juveniles harbored fewer mites than adults and the difference was also significant between the 1-year-old birds and those over 1 year old in the breeding population. The number of mites did not change after the second year of life of the birds. We hypothesize that the difference in abundance of mites between the age classes can be explained by the low reproductive potential of the mites, which are not able to populate the exploitable space until the second year of life of the host. Alternatively, young birds might provide fewer resources than old birds. The significant negative association between the number of mites and the laying date of female barn swallows seems to support the conclusion that the abundance of mites is condition-dependent. Because there was no relationship between other condition indices for males and females and number of mites, further research is needed to confirm this conclusion.