We describe a case in which occupation by the European Tree Frog Hyla arborea caused the nest failure of a pair of Eurasian Reed Warblers Acrocephalus scirpaceus. The frog was observed several times sitting on the reed warblers clutch, on the day when the third egg was laid, thereby preventing the parental birds from incubating. Although small frog species have been reported to rest during the day in bird nests (mainly empty ones), to our knowledge this is the first published report providing evidence for avian nest failure due to nest occupation by an amphibian species.