Effect of introduced parasites on the survival and microbiota of nestling cactus finches (Geospiza scandens) in the Galápagos Islands

Introduced parasites and pathogens have colonized the Galápagos Islands of Ecuador. For example, a parasitic nest fly, Philornis downsi, was introduced to the Galápagos and has since caused significant nestling mortality for some endemic Darwin’s finches. However, some larger bodied species of birds in the Galápagos, including vegetarian finches (Platyspiza crassirostris) and Galápagos mockingbirds (Mimus parvulus), can be less affected by the parasite. Our study explores the effects of P. downsi on a small population of common cactus finches (Geospiza scandens) across two breeding seasons on San Cristóbal Island, Galápagos. Specifically, we experimentally manipulated P. downsi abundance and characterized the gut and skin microbiota, and survival of nestling finches. Because cactus finches are medium-sized birds and small- and medium-sized birds are generally negatively affected by P. downsi, we predicted that the parasite would have a similar detrimental effect on fledging success. We also determined the effect of P. downsi on the microbiota because other studies have shown that the bacterial community can be beneficial for host health (e.g., conferring immunity and nutrient absorption). Although the overall skin and gut microbiota differed, we found that parasite treatment did not affect the microbiota in either tissue. However, nestlings from parasitized nests had significantly lower fledging success (25%, on average) compared to nestlings from non-parasitized nests (79%). We found that, similar to other medium-sized species, cactus finches are negatively affected by P. downsi, which could have implications for their risk of extirpation from the island. The results of this study should be considered when managing this population of concern.