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Predators and parasites

Nesting colonies attracted large numbers of predators, including American minks (Neogale vison), long-tailed weasels (Neogale frenata), American martens (Martes americana), and raccoons (Procyon lotor) that preyed on eggs and nestlings, birds of prey, such as owls, hawks, and eagles that preyed on nestlings and adults, and wolves (Canis lupus), foxes (Urocyon cinereoargenteus and Vulpes vulpes), bobcats (Lynx rufus), American black bears (Ursus americanus), and cougars (Puma concolor) that preyed on injured adults and fallen nestlings. Hawks of the genus Accipiter and falcons pursued and preyed upon pigeons in flight, which in turn executed complex aerial maneuvers to avoid them; Cooper's hawk (Accipiter cooperii) was known as the "great pigeon hawk" due to its successes, and these hawks allegedly followed migrating passenger pigeons. While many predators were drawn to the flocks, individual pigeons were largely protected due to the sheer size of the flock, and overall little damage could be inflicted on the flock by predation. Despite the number of predators, nesting colonies were so large that they were estimated to have a 90% success rate if not disturbed. After being abandoned and leaving the nest, the very fat juveniles were vulnerable to predators until they were able to fly. The sheer number of juveniles on the ground meant that only a small percentage of them were killed; predator satiation may therefore be one of the reasons for the extremely social habits and communal breeding of the species.

Two parasites have been recorded on passenger pigeons. One species of phtilopterid louse, Columbicola extinctus, was originally thought to have lived on just passenger pigeons and to have become coextinct with them. This was proven inaccurate in 1999 when C. extinctus was rediscovered living on band-tailed pigeons. This, and the fact that the related louse C. angustus is mainly found on cuckoo-doves, further supports the relation between these pigeons, as the phylogeny of lice broadly mirrors that of their hosts. Another louse, Campanulotes defectus, was thought to have been unique to the passenger pigeon, but is now believed to have been a case of a contaminated specimen, as the species is considered to be the still-extant Campanulotes flavus of Australia. There is no record of a wild pigeon dying of either disease or parasites.