Dry year does not reduce invasive parasitic fly prevalence or abundance in Darwin's finch nests

Abstract

The recent introduction of the parasitic nest fly Philornis downsi (Diptera: Muscidae) to the Galápagos Islands poses a serious threat to the bird species it infests, including Darwin’s finches. Variation in climatic conditions, such as rainfall or drought, may influence fly populations and their effect on birds. We monitored the abundance of P. downsi in an extremely dry year in nests of the medium ground finch (Geospiza fortis). We fumigated nests to compare the reproductive success of birds in nests with and without flies. Neither the prevalence nor the abundance of P. downsi decreased significantly in the dry year compared with an earlier wet year for which we have already published data. Very few birds bred successfully under the dry conditions, independent of parasite prevalence and abundance. The low reproductive success of the finches presumably reflected limited food resources rather than parasites. Our sample sizes were low because few birds attempted to breed in the dry year. Nevertheless, our data indicate that P. downsi is capable of withstanding the extreme climatic fluctuations characteristic of the Galápagos Islands, which may contribute to the invasiveness of this parasite.