of nearly extinct birds, such as the kakapo (Strigops habroptilus) and the takahe (Porphyrio hochstetteri), have been enough to keep those species extant to the present. The combined effects of intense hunting and deforestation has been referred to as a "Blitzkrieg" against the passenger pigeon, and it has been labeled one of the greatest and most senseless human-induced extinctions in history. As the flocks dwindled in size, the passenger pigeon population decreased below the threshold necessary to propagate the species, an example of the Allee effect. The 2014 genetic study that found natural fluctuations in population numbers prior to human arrival also concluded that the species routinely recovered from lows in the population, and suggested that one of these lows may have coincided with the intensified hunting by humans in the 1800s, a combination which would have led to the rapid extinction of the species. A similar scenario may also explain the rapid extinction of the Rocky Mountain locust (Melanoplus spretus) during the same period. It has also been suggested that after the population was thinned out, it would be harder for few or solitary birds to locate suitable feeding areas. In addition to the birds killed or driven away by hunting during breeding seasons, many nestlings were also orphaned before being able to fend for themselves. Other, less convincing contributing factors have been suggested at times, including mass drownings, Newcastle disease, and migrations to areas outside their original range. The extinction of the passenger pigeon aroused public interest in the conservation movement, and resulted in new laws and practices which prevented many other species from becoming extinct. The rapid decline of the passenger pigeon has influenced later assessment methods of the extinction risk of endangered animal populations. The International Union for Conservation of Nature (IUCN) has used the passenger pigeon as an example in cases where a species was declared "at risk" for extinction even though population numbers are high. Naturalist Aldo Leopold paid tribute to the vanished species in a monument dedication held by the Wisconsin Society for Ornithology at Wyalusing State Park, Wisconsin, which had been one of the species' social roost sites. Speaking on May 11, 1947, Leopold remarked: Potential resurrection of the species Today, more than 1,532 passenger pigeon skins (along with 16 skeletons) are in existence, spread across many institutions all over the world. It has been suggested that the passenger pigeon should be