Group-Living and Infestation by Ectoparasites in Passerines

Using data from an 11-year field study, I determined whether group-living passerine species showed greater infestation by contact-transmitted ectoparasites (feather mites) than sympatric, solitary species. Differences in infestation by mobile hippoboscid flies between group-living and solitary passerine species were also examined. Among the 45 species included in the analysis, feather mite prevalence was significantly greater on group-living species than on solitary ones. Two other factors investigated, bird size and migratory habit, had no effect on infestation levels. Similar results were obtained when the analysis was performed on all species outside the most species-rich family (Fringillidae), or only within the family Fringillidae itself. An analysis at the family level also suggested greater feather mite infestations in families made up only of group-living species as opposed to families including only solitary species; however, the difference was not significant. No differences were observed in hippoboscid fly abundance between group-living and solitary passerines, in all analyses at either the species or family level. These results suggest that individuals of group-living species incur a greater risk of acquiring contact-transmitted ectoparasites than individuals of solitary species, a cost which may not be negligible.