Invasive parasites can have devastating effects on native host species that lack effective defense mechanisms. For example, the invasive parasitic nest fly Philornis downsi severely reduces the breeding success of Darwin’s finches in the Galapagos Islands. In contrast, Galapagos mockingbirds are tolerant hosts that often suffer no reduction in fitness, even when parasite density is identical. Tolerance is good for mockingbirds, but potentially bad for finches, assuming that mockingbirds serve as reservoir hosts that amplify the threat to vulnerable hosts. I describe fieldwork my students and I are conducting to test the reservoir host hypothesis.