Priority effects influence invasive parasite interactions in Darwin's finches

Invasive parasites are a major threat to naive, island hosts, who often deal with multiple parasite taxa (hereon, co-parasitism). These parasite taxa can positively or negatively affect each other through the host’s immune system, which can depend on the order of exposure (i.e., priority effects). Several parasite taxa, such as the avian vampire fly (Philornis downsi) and avian pox virus, were introduced to the Galápagos Islands and negatively affect endemic birds, such as Darwin’s finches. Although finches can be parasitized by both taxa, the effect of co-parasitism on finches and the dynamics between parasite taxa are unknown. For our study, we experimentally manipulated vampire fly abundance in the nests of small ground finch (Geospiza fuliginosa), then determined the effect of treatment on the prevalence of avian pox in nestlings. We also determined whether nestlings parasitized by flies and/or pox differed in body condition, hemoglobin levels (proxy for blood loss), and survival. We found that nestlings from fly-parasitized nests were less likely to test positive for pox than nestlings from nests with flies. Additionally, nestlings with low (0) and high (>40) fly abundances were more likely to test positive for pox compared to nestlings with intermediate abundances (10-30). Although co-parasitism did not affect nestling condition and survival, co-parasitized nestlings had lower hemoglobin levels than nestlings with no or only one parasite taxa. Overall, our study supports the presence of priority effects, in which flies help nestlings resist pox infection until fly numbers are too high for nestlings to defend themselves.