Understanding the factors affecting parasite aggregation in natural host populations is one of the central questions in parasite ecology. While different biological mechanisms giving rise to aggregation have been documented in the literature, the role of established parasites in vector attraction and its importance in determining clumped parasite distributions has received less attention. In a two-year field study we evaluated the importance of a bird vector, Mimus thenca (Mimidae), on the aggregation dynamics of the holoparasitic mistletoe Tristerix aphyllus on its cactus host, Echinopsis chilensis. Removal of T. aphyllus from cacti decreased the number of visits and the time spent by the bird vector, which resulted in a 3.5-fold lower seed deposition of the mistletoe on experimental hosts than on control hosts. Vector preference, however, was not the only factor affecting aggregation in this system. Spine length of the cactus acted as a first line of defense against parasitism by discouraging bird perching on top of host columns. While heavily parasitized hosts received more seeds than unparasitized hosts, spines counteracted this effect. These results provide field evidence that parasite aggregation results from the balance between vector behavior and host resistance traits.