Parental care in Darwin's finches

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.