Encountering suitable hosts is key for parasite success. In a natural system involving a parasitic fly and its multiple bird hosts there are profound differences in host quality. The Great Kiskadee tolerates and does not invest in resisting the infection, which makes it an optimal host. Alternative hosts are frequently used, but whilst some of them may be good options, others are bad alternatives (they resist efficiently or die). Here we examined the host selection processes that drive parasite dynamics in this system with a thorough longitudinal study under natural conditions. We found that host selection is strongly driven by availability of quality hosts: the parasite chooses suboptimal hosts only when better alternatives are not sufficiently available. This adds evidence from a natural system that hosts are chosen as a function of their profitability, and shows that host selection by a parasite may be plastic and context-dependent.