Life-stage and sex influence Philornis ectoparasitism in a Neotropical woodpecker (Melanerpes striatus) with essential male parental care

The nestlings of many Neotropical bird species suffer from Philornis (Diptera: Muscidae) ectoparasitism. While nestlings are typically considered the intended targets, recent work indicates that Philornis infest adult birds more frequently than previously appreciated, yet few studies have concurrently surveyed nestlings and adults for Philornis in the same population. Over six field seasons (2012-2017), I documented the presence of current or recent subcutaneous Philornis infestations on adult and nestling Hispaniolan Woodpeckers Melanerpes striatus from the same population in the central Dominican Republic. I tested the following three non-mutually exclusive hypotheses regarding occurrence of Philornis on adult birds: (1) nestlings are more vulnerable to Philornis parasitism than adults, (2) nesting is associated with Philornis parasitism in adults, and (3) Philornis parasitism is associated with incubation and brooding investment. While nestling and adult woodpeckers exhibited similar prevalence of parasitism, parasitized nestlings hosted on average 3.5 times more Philornis wounds (larvae plus empty wounds) than parasitized adults. Nesting per se was not significantly associated with parasitism among adults, as breeding and non-breeding adults showed similar prevalence and intensity. However, nests with Philornis-infested young were significantly more likely to have one or both parents also be infested in contrast to nests with infestation-free young. Furthermore, adult males, which perform overnight incubation and brooding, were significantly more likely to be parasitized than adult females. This last result supports the hypothesis that incubation and brooding investment increase the risk of Philornis parasitism for adults, but this conclusion is complicated by the lack of an association between parasitism and nesting status. Together, these results raise questions about the degree of host life-stage specialization and whether adult parasitism is incidental or part of an alternative parasitic strategy for Philornis.