In 1993-1995 artificial nests with attached model eggs were put into territories that were known to have been occupied by male great reed warblers, Acrocephalus arundinaceus, in previous years. Because the eggs were made of soft plasticine, predators left peckmarks in them and this enabled us to identify predators by comparing peckmarks with reference marks made by Various species. Previous field data had suggested that infanticidal behaviour existed in our study population, as nests of primary females suffered a three times higher rate of nest loss during the egg-laying period than nests of secondary and monogamous females. The presence of infanticide was supported by the experiment. Small peckmarks resembling those of a great reed warbler occurred almost exclusively in territories occupied by great reed warblers, in particular when a new female settled in the territory. The newly settled females built nests closer to depredated than non-depredated nests. That small peckmarks occurred when new females settled strongly suggests that it is secondary female great reed warblers that commit infanticide on eggs of primary females. Females of low harem rank are expected to gain from infanticidal behaviour because a low ranked female gets a higher proportion of male parental investment when the nest of the primary female fails. (C) 1997 The Association for the Study of Animal Behaviour.