Abstract
Host species have evolved different strategies for dealing with parasites. ‘Vulnerable’ hosts, which suffer reduced fitness from parasites, are under selection to evolve defences. ‘Tolerant’ hosts, by contrast, avoid reductions in fitness by mitigating parasite damage. When tolerant and vulnerable hosts co‐occur, tolerant hosts may pose an indirect threat to vulnerable hosts by serving as a source of parasite infestation. Avian vampire flies ( Philornis downsi ) are introduced parasites of Galapagos mockingbirds ( Mimus parvulus ) and Darwin's finches. Mockingbirds are relatively tolerant, while finches are more vulnerable. We tested the indirect threat hypothesis using a population genomics approach to estimate transmission of vampire flies from mockingbirds to finches on Santa Cruz Island, Galapagos. Mockingbirds nested earlier than finches, which increased their potential threat to finches. We sequenced 136 whole genomes of avian vampire flies and did kinship analysis to compare the relatedness of flies in mockingbird and finch nests over the course of the breeding season. Our data show that flies in source mockingbird nests are more often related to flies in finch nests than to flies in mockingbird nests later in the season. This pattern may be explained by the distance between nests, as finch nests are often closer to mockingbird nests than mockingbird nests are to each other. These results are consistent with the hypothesis that tolerant mockingbirds pose an indirect threat to more vulnerable finches.