Modern conservation efforts tend not to focus on individual species, but rather on the entire ecosystem of a species in peril. Many ecological factors can affect a species’ ability to maintain healthy populations. Parasites, which derive nutrients at the expense of their hosts, can reduce host fitness and limit population growth, acting as biological controls in healthy ecosystems. The negative impacts of parasites on their hosts can be exacerbated by climate change and anthropogenic land-use practices in ways that may limit recovery or drive host species to extinction. Introduced parasitic nest flies in the genus Philornis (Diptera: Muscidae) are threatening the extinction of bird species in the Galápagos, yet almost nothing is known about Philornis-host ecology in systems where the fly is native. To fill this knowledge gap, we examined the ecological relationship between the parasitic nest fly Philornis pici and its host, the Critically Endangered Ridgway’s hawk (Buteo ridgwayi) in Los Haitises National Park in the Dominican Republic. We excluded nest flies from some Ridgway’s hawk broods and compared fledging success with that of control broods, from which flies were not excluded. Treated young had an 89% lower infestation rate and were 179% more likely to fledge than were untreated (control) young. Further, because of the recent history of deforestation in the region, we measured biotic variables around untreated Ridgway’s hawk broods and compared these values with abundance and prevalence of nest fly infestation in nestling hawks. We found P. pici infestation was negatively associated with grass-cover around hawk nests, which suggests that managing certain aspects of land cover may be a way to mitigate parasitism levels of Ridgway’s hawks. Our work is novel in that we offer the first measurable impact of nest fly infestation on survival or productivity in a non-passerine host. Our findings suggest that P. pici parasitism of hawk nestlings could be a factor in the decline of the Ridgway’s hawk.