Avian brood parasitism represents an ideal system to study co-evolution because many adaptations and counter-adaptations have evolved during the co-evolutionary struggle between the parasites and their hosts. Although this fascinating phenomenon has been intensively explored, there are still many unresolved questions both on the side of the parasite and the host. In this project, we will focus on understudied areas of parasite breeding biology, such as host selection, offspring sex allocation and survival, as well as on reproductive success of the parasite; and how these traits are affected by host quality. Because it is logistically extremely difficult to investigate breeding biology of the parasites due to their elusive lifestyle, we will apply progressive methodological approaches that account for individual identity. As host choice and individual fitness have fundamental consequences for both parasite and host populations, our research will help to better understand the dynamics of host-parasite interactions and thus co-evolutionary processes in general.