Understanding how transmission influences parasite population structure and virulence in understudied non-human pathogens is crucial for general models of host-parasite coevolution and our ability to cope with emerging diseases. Therefore, I propose a study to reveal coevolutionary and population dynamics and their underlying mechanisms within and between hosts of parasites in wild bird populations. To achieve this, I will make use of the unique opportunities offered by raptors, an intensively studied group across Europe, in combination with the first developed set of population genetic markers for their specialised malaria-like parasites.My preliminary work resulted in a draft genome for Leucocytozoon, a blood parasite of raptors, which allows me to use microsatellite and parasite antigenic markers and reconstruct transmission pathways. I will first analyse these in one population of Common Buzzards Buteo buteo, where over 1900 nestlings i.e. all naïve hosts in the area, have been sampled over the last 11 years, and data on host phenotype and transmission locations are available. A cDNA microarray developed for both buzzard and parasite, and multiple samples taken in the course of infections will provide an unparalleled picture of the host-parasite molecular arms race, its dependence on population connectivity, and its effects on host fitness. A collaboration network with raptor research and conservation groups, and animal rescue centres across Europe will also be initiated. It will provide samples from different raptor species and populations and allow me to find scale-dependent patterns of parasite population dynamics and discover how gregariousness and other host life-history traits determine parasite population structure and virulence.In the course of this study, I will acquire combinations of state-of-the-art molecular techniques, will use my networking skills and improve my complementary skills enabling me to lead my own research group in the near future.