Selection should act on parental care and favour parental investment decisions that optimise the number of offspring produced. Such predictions have been robustly tested in predation risk contexts, but little is known about parental care investment trade-offs under conditions of parasitism. The avian vampire fly, Philornis downsi (Diptera: Muscidae), is a myasis-causing ectoparasite accidentally introduced to the Galápagos Islands, and one of the major causes of failure in Darwin’s finch nests. With an 11-year dataset spanning 21 years, we examine the relationship between parental care behaviours and the number of fly larvae and pupae in Darwin’s finch nests. We do so across three host species (Camarhynchus parvulus, C. pauper, Geospiza fuliginosa) and one hybrid Camarhynchus group. Nests with longer female brooding duration had fewer parasites, and this effect intensified with higher male food delivery to chicks. Neither male age nor number of nest provisioning visits alone were directly associated with parasite burden. While the causal mechanisms remain unknown, we provide the first empirical study showing that female in-nest attendance duration is negatively related to ectoparasite burden. We predict selection for coordinated host male and female behaviour to reduce gaps in nest attendance, especially under conditions of novel and introduced ectoparasites.

Focal observations of nesting behaviour were made at Darwin's finch nests. After the nesting outcome, the number of Philornis downsi larvae per nest were counted.

The raw data are presented per nest per row; column headings describe the column contents. Sheet 2 describes the column labels. Data on nesting behaviour at Darwin's finch nests were collected from one hour focal observations of individually monitored nests on Santa Cruz and Floreana Islands, Galapagos Archipelago, between 2000 and 2020.