Parasites can influence evolutionary patterns in their hosts by generating a mosaic of selection pressures across populations. Variations in parasite prevalence and host-specificity can determine how parasites are distributed among hosts and, ultimately, how parasite interactions influence host biogeography. However, our understanding of how host-parasite interactions influence biogeographic patterns is limited by a lack of appropriate model systems. Avian malaria parasites (Plasmodium and Haemoproteus; Haemosporida) are a diverse, globally distributed group of vector-borne blood parasites. Because avian malaria prevalence and community composition can vary across host populations, these parasites have potential to impose spatially divergent selective pressures in birds. This thesis generates new perspectives on the evolutionary importance of avian malaria by exploring patterns in malaria prevalence, diversity and host-specificity in the under-studied South Pacific. Specifically, I sample wild birds for malaria to test whether (a) these parasites conform to classic global and island biogeography patterns, and (b) whether variation in parasite prevalence and host-specificity may influence the distributions and evolutionary trajectories of their avian hosts.