Priority Effects Shape Interactions Between Invasive Parasites in Darwin's Finches of the Galápagos Islands

ABSTRACT Invasive parasites are a major threat to naïve, island hosts. Furthermore, hosts often deal with multiple parasite taxa (hereon, co‐parasitism), which can directly compete with each other or indirectly interact through the hosts' immune system (i.e., priority effects). Several introduced parasite taxa, such as the hematophagous avian vampire fly ( Philornis downsi ) and avian pox virus, have established in the Galápagos Islands and negatively affect endemic birds, such as Darwin's finches. The overall objective of our study was to understand the dynamics between avian vampire flies and avian pox in small ground finches ( Geospiza fuliginosa ). First, we tested and found the presence of two avian pox strains in nestlings, their mothers, and larval files. Our results suggest that avian pox is not transmitted vertically because strains did not match between nestlings and their mothers; however, larval flies have transmission potential because their strains matched their respective host. Fly abundance was also experimentally manipulated to determine the effect of fly parasitism on pox prevalence in nestlings. We found that vampire flies, depending on their abundance, may inhibit and facilitate pox establishment. Nestlings without flies were more likely to test positive for pox than nestlings with intermediate fly abundances, suggesting that priority effects inhibit or delay the establishment of pox. However, priority effects appeared to shift to facilitation of pox establishment in nestlings with high fly abundances. Our study presents evidence for inhibitory and facilitatory priority effects of early‐life fly parasitism on risk of pox infection in nestling finches.