Intraspecific brood parasitism (laying eggs in another's nest) occurs widely in colonial cliff swallows (Passeriformes: Hirundinidae: Hirundo pyrrhonota). In colonies consisting of more than ten nests, up to 24 percent of the nests were sometimes parasitized by colony members. Laying eggs in a conspecific's nest may be a benefit of coloniality for parasitic individuals and simultaneously may represent a cost to host individuals within the same colony.