Video analysis of host-parasite interactions in nests of Darwin's finches

Parasites place their hosts under strong selection for adaptive traits that increase parasite resistance. The initial impact of invasive parasites has rarely been observed and can be particularly strong on naive hosts with limited prior exposure to parasites. Philornis downsi is an introduced fly to the Galapagos Islands whose parasitic larvae cause high mortality in nestlings of Darwin's finches. We used a within-nest camera system and nest monitoring data to examine this new host-parasite interaction in the wild. Many P. downsi flies entered finch nests with incubated eggs or nestlings but only when parent finches were not present. Parasitic P. downsi larvae were observed to emerge from the nest base at night to feed both internally and externally on nestlings. Adult and nestling Darwin's finches exhibit grooming and avoidance behaviours in the presence of P. downsi parasites. Specifically, in nests with high parasite intensity, nestlings increased self-preening behaviour, ate larvae and stood on top of one another. Female finches probed into their nestling's nares (first instar larvae reside in the nares) and probed into the nest base (second and third larvae reside in the nest base during the day). These findings shed light on the emergence of anti-parasite behaviour as well as host-parasite relationships after recent parasitism in a naive host.