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Rare species, particularly those in inaccessible habitat, can go years without being observed. If we are to allocate conservation resources appropriately to conserving such species, it is important to be able to distinguish "rare" from "extinct."  Criteria for designating extinction, however, tend to be arbitrary or vaguely defined. This designation should not be made unless the search effort has been sufficient to yield a high degree of confidence that the species is in fact absent. We develop models to assess the probability of extinction and the search effort necessary to detect an individual in a small population. We apply these models to searches for nine potentially extinct Hawaiian forest birds and for the Ivory-billed Woodpecker (Campephilus principalis) in intensively searched areas in Arkansas. The Hawaiian forest bird survey was extensive, providing excellent information on population sizes and habitat associations of species encountered during the survey. Nonetheless, we conclude that the survey effort was not sufficient to conclude extinction (p > 0.90) for populations of 10 or fewer individuals for those species that were not encountered during surveys. In contrast, our analysis for Ivory-billed Woodpeckers suggests that, unless there were actually two or fewer birds present, the search effort was sufficient to conclude (p > 0.95) that Ivory-billed woodpeckers were not  present in the intensively searched area. If one assumes distributions other than uniform, there is a greater chance that Ivory-billed Woodpeckers may  persist in the intensively searched areas. Conclusions regarding occupancy of suitable habitat throughout the rest of the former range will require similarly intensive survey efforts. The degree of confidence in the absence of the Ivory-billed Woodpecker depended in part on our assumptions about the distribution of birds in the search area. For species with limited detection distance and small populations, a massive search effort may be required to conclude with confidence that a species is unlikely to be extant

When is an &#8220;Extinct&#8221; Species Really Extinct? Gauging the Search Efforts for Hawaiian Forest Birds and the Ivory-Billed Woodpecker

When is an “Extinct” Species Really Extinct? Gauging the Search Efforts for Hawaiian Forest Birds and the Ivory-Billed Woodpecker