During a study of bird nesting in SW. Anatolia, 125 man-made nest boxes have been installed, 3 m high, in the trees. These nest boxes have been occupied by several bird species (Phoenicurus phoenicurus, Parus major, Parus ater, Sitta krueperi, Certhia brachydactyla). Of the 48 Redstart nests, 19 (40 %) had been successfully invaded by the bumblebee Bombus niveatus vorticosus. This invasion occurred at different times in the nest building cycle: - during the building of the nest, - during the incubation or - after the hatching of the young. Once installed in the nest of the Redstart, the bumblebee does not directly attack the bird, but disturbs it by continuously rearranging the nesting material and by covering the bird's brood. Eventually, the bird deserts its nest and brood. The Redstart does not display any agonistic behaviour towards the bumblebee. Nests of other bird species are never invaded.