Can a Blood-Feeding Ectoparasitic Fly Affect Songbird Migration? Examining Body Condition and Fat Reserves of Five Bird Species in Relation to Hippoboscid Fly Parasitism

Abstract

Background. Migratory birds are often faced with the challenge of undertaking long-distance journeys while harboring parasites. Objective. I investigated the possibility that Hippoboscid flies (Diptera, Hippoboscidae), blood-feeding ectoparasites, would be associated with reduced body condition or fat reserves of migratory songbirds. Methods. I mist-netted songbirds on an island off the southern coast of Nova Scotia, Canada during fall 1996 and noted the presence of hippoboscid flies on five focal species. Of 731 individuals, 60 (8.2%) had flies. Results. In statistical models that considered species and parasitism, I found no effect of flies on body condition, but birds with flies had significantly reduced fat reserves. This pattern was found in all species to varying degrees. Conclusions. While these results do not confirm causality, they are consistent with the idea that parasitized birds suffer a cost to their migratory preparations. It is unclear whether the birds were in a premigratory stage or actually migrating, although reductions in fat in either case would be costly. This study highlights how host-parasite interactions must be studied in multiple life stages to gain a complete understanding of their nature.