Infanticide, the killing of offspring by adult conspecifics, has been demonstrated in many insect, mammal and bird species. In contrast to selective pressures influencing infanticide in other species, egg destruction in birds is thought to primarily represent interference competition for food, nest sites or predator-free nesting space. In the case of the great reed warbler, two opposing hypotheses exist for the explanation of its egg destruction behaviour. Our study tested these hypotheses separately by manipulating the presence of real conspecific, familiar and unfamiliar eggs and nests inside polygynous and monogamous great reed warbler territories. Out of 147 experimental nests placed in the vicinity of 49 active great reed warbler nests, only conspecific eggs were preyed upon by great reed warblers. Furthermore, significantly more great reed warbler nests were destroyed in polygynous than monogamous territories. These results support the hypothesis that egg destruction behaviour in this species is motivated intraspecifically and most probably intrasexually.