Parasite diversity on remote oceanic archipelagos is determined by the number and timing of colonizations, and in-situ diversification rate. In this study we compare intra-archipelago diversity of two hemosporidian parasite genera, Plasmodium and Leucocytozoon, infecting birds of the Mascarene archipelago. Despite the generally higher vagility of Plasmodium parasites, we report a much lower diversity of Plasmodium cytochrome-b haplotypes in the archipelago compared to Leucocytozoon. Using phylogenetic data, we find that this difference in diversity is consistent with differences in the timing and the number of colonizations, while rates of diversification do not vary significantly between the two genera. The prominence of immigration history in explaining current diversity patterns highlights the importance of historical contingencies in driving disparate biogeographic patterns in potentially harmful blood parasites infecting island birds.