INFANTICIDE IN GREAT REED WARBLERS - SECONDARY FEMALES DESTROY EGGS OF PRIMARY FEMALES

In 1993-1995 artificial nests with attached model eggs were put into t erritories that were known to have been occupied by male great reed wa rblers, Acrocephalus arundinaceus, in previous years. Because the eggs were made of soft plasticine, predators left peckmarks in them and th is enabled us to identify predators by comparing peckmarks with refere nce marks made by Various species. Previous field data had suggested t hat infanticidal behaviour existed in our study population, as nests o f primary females suffered a three times higher rate of nest loss duri ng the egg-laying period than nests of secondary and monogamous female s. The presence of infanticide was supported by the experiment. Small peckmarks resembling those of a great reed warbler occurred almost exc lusively 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 smal l peckmarks occurred when new females settled strongly suggests that i t is secondary female great reed warblers that commit infanticide on e ggs 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 f emale fails. (C) 1997 The Association for the Study of Animal Behaviou r.