The Diptera occurring in various kinds of bird nests were investigated in the Japanese island of Hokkaido from 1985 to 1989. A total of 4623 Diptera, consisting of 23 species of eight families (Anthomyiidae, Fannidae, Muscidae, Calliphoridae, Heleomyzidae, Sphaeroceridae, Sepsidae, and Stratiomyiidae), was reared from 69 natural nests of 13 bird species of eight families (Strigidae, Picidae, Paridae, Sittidae, Sturnidae, Ploceidae, Laniidae, and Columbidae). Of 23 species of flies reared, 12 were new records from bird nests. An ectoparasite of birds, Protocalliphora maruyamensis Shinonaga and Kano, frequently emerged from the nests of hole-nesting birds of Paridae, Sturnidae, and Ploceidae, but it did not emerge from the nests of the hole-nesting birds Picoides major and P. minor of Picidae. The nests of Streptopelia orientalis yielded the most species of flies (12). Myospila meditabunda (Fabricius) and Mydaea urbana (Meigen), whose larvae are predacious, proved able to live in the bird nests. Among the scavengers, Potamia littoralis (R.-D.) was the most dominant and occurred in the nests of nine of 10 hole-nesting bird species. Sphaerocerid, sepsid, and stratiomid flies occurred in the nests of S. orientalis in which large quantities of bird feces had accumulated. The factors concerning the occurrence of flies in the nests are discussed.