Effect of an introduced parasite in natural and anthropogenic habitats on the breeding success of the endemic little vermilion flycatcher Pyrocephalus nanus in the Galápagos

In the Galápagos Islands many endemic bird species, including the emblematic Darwin's finches, show significant declines in population size. The endemic little vermilion flycatcher Pyrocephalus nanus, classified as vulnerable with high extinction risk, is strongly declining and broods regularly fail. We investigated multiple causes for breeding failure by comparing breeding success, infestation intensity by the hematophagous larvae of the introduced parasitic fly Philornis downsi, predation, parental food provisioning rate and prey attack rates as indicators of food availability at three study sites differing in anthropogenic habitat alterations: Alcedo on Isabela Island with its pristine habitat remote from human settlements, El Cura, also on Isabela Island but dominated by farmland, and Mina Roja on Santa Cruz Island, a site highly altered by introduced invasive plant species, mainly the blackberry. To test for the causal role of parasitism, we reduced the number of P. downsi larvae in half the nests at each site on Isabela Island, and used the other nests as control. When infestation intensity was experimentally reduced, both breeding success and food provisioning rates increased significantly in El Cura, but not in Alcedo, where breeding success and food provisioning rates were overall higher and infestation intensity lower than in El Cura. In the very small population of the little vermilion flycatcher in Mina Roja most nests were abandoned during the incubation phase before nests were infested by P. downsi. Mammalian predation played a minor role in brood loss at all three study sites. Our experimental study demonstrates that the recently introduced parasitic fly significantly affects breeding success of an endangered endemic bird species, and suggests that the effects are modulated by natural levels of P. downsi infestation and habitat-related rates of food provisioning. Conservation measures should include P. downsi control combined with creating and maintaining open foraging areas.