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We measured how fighting, mate-guarding, ectoparasitism, and annual reproductive success varied with colony size in Purple Martins (Progne subis) in Tulsa County, Oklahoma, in 1997. Fights bird^-1 hr^-1 changed significantly with colony size, peaking in intermediate-sized colonies. Intensity of mate-guarding by males increased significantly with colony size. Ectoparasitism of nestlings by the martin mite (Dermanyssus prognephilus) increased significantly with colony size, but nestling body mass declined with mite load only for broods of six nestlings. Reproductive success did not vary significantly with colony size. Purple Martin colonies probably do not routinely reach large enough sizes for the costs of fighting or ectoparasitism to be important. These birds apparently do not experience direct benefits of coloniality, and groups likely form in response to limited nesting sites. The pattern in reproductive success …