The selective advantage of Müllerian mimicry in nature was investigated by releasing live mimetic and nonmimetic butterflies close to wild, aerial-hunting tropical kingbirds (Tyrannus melancholicus) and cliff-flycatchers (Hirundinea ferruginea) in three Amazon habitats (rain forest, a city, and “canga” vegetation). Only mimetic butterflies elicited sight-rejections by birds, but protection conferred by mimicry was restricted to sites in which both predators and mimics co-occurred, as in the case of six mimicry rings at a forest site and two at a city site. Most other Müllerian mimics released at city and canga vegetation were heavily attacked and consumed by birds. These results appear to reflect the birds' previous experiences with resident butterfly faunas and illustrate how birds' discriminatory behavior varied among habitats that differed in butterfly species and mimicry ring composition.