Effects of the introduced parasite Philornis downsi on nestling growth and mortality in the medium ground finch (Geospiza fortis)

Invasive species have the potential to detrimentally affect native ecosystems by out competing or directly preying upon native organisms, and have been implicated in the extinction of endemic populations. One potentially devastating introduced species in the Galápagos Islands is the parasitic fly Philornis downsi. As larvae, P. downsi parasitize nestling birds and have been associated with high nestling mortality and reduced growth rates. Here I document nestling growth and mortality in a bimodal population of the medium ground finch, Geospiza fortis. Observations were conducted over three years, and under variable ecological conditions. Annual parasite prevalence in nests ranged from 64% to 98%, and nestling mortality in nests with parasites ranged from 16% to 37%. Parasite load and parasite load per nestling follow a skewed distribution with many nests having relatively few parasites, and few nest having many. Parasite load, however, was not correlated with onset of breeding, clutch size, the number of nestlings, nestling survival or fledgling success. Parasite load per nestling, on the other hand, was correlated with clutch initiation date and the proportion of nestlings that died in parasitized nests. Neither nestling size nor growth rate differed between parasitized and unparasitized nests. In addition, male and female beak morphology was not correlated with parasite load, breeding variables or nestling survival. Thus, while overall mortality due to parasitism is high, ecological conditions and possible host defenses may potentially counter some of the detrimental affects of P. downsi on nestling size and growth. These results taken together suggest that parasitism of P. downsi larvae on nestling G. fortis has the potential to lead to large population declines.