Aggression between chicks and fledglings, at times ending in siblicide and cannibalism, has been mostly studied among nest mates. It is frequent among colonial-nesting birds and is usually related to competition for limiting resources (e.g., food and space) and competitive disparities between siblings, among other factors. We report three observations of full-grown Peruvian Pelican (Pelecanus thagus) fledglings attacking unrelated conspecific nestlings at a breeding colony in central Chile. One case ended in cannibalism. Five elements were common in all three cases: (1) nestlings that were attacked were left unattended by their parents in the nest, (2) nestlings that were attacked were newly hatched (up to 5 days of age), (3) aggressors were full-grown fledglings (60–75 days of age), (4) aggressive fledglings always attacked nestlings in groups, and (5) all cases were observed late in the breeding season. We suggest aggression toward and cannibalism of nestlings by fledglings are opportunistic behaviors, based on the opportunity of finding unattended nests and are triggered by food deprivation, although hormonal mechanisms may also be involved. Our observations constitute the first report of aggression and cannibalism by Peruvian Pelican fledglings.