Grassquits adjust parental care in broods parasitized by native nest flies

The parasitic nest flies Philornis spp. are ectoparasites associated with a large number of Neotropical bird species. Little is known about how flies locate birds' nests and how parents respond to native or subcutaneous parasitic flies. Here, we used blue-black grassquits (Volatinia jacarina) to test (i) if parental care in early egg and nestling stages could predict parasitism status in late nestling stages and (ii) if parental care differed between parasitized and non-parasitized nests. In general, the proportion of time parents were on the nest (both incubation and nestling periods) did not predict later Philornis parasitism status. These results suggest that flies do not use the presence of parents at the nest to locate nestlings. Parents visited the nest to feed nestlings more frequently in parasitized nests, suggesting that parents compensate for presumed nestling energy loss due to Philornis parasitism. Parents also spent more time on nest maintenance in parasitized nests, which may be an attempt to remove larvae from nest walls or repair the nest damaged by larvae displacement. Finally, the time spent brooding did not vary with parasitism status. Grassquit responses to Philornis parasitism provide a new perspective relative to the evolution of effective defense strategies against native parasites.