The Tabon Cave complex is named after the "Tabon bird" (Tabon scrubfowl, "Megapodius cumingii"), which deposited thick hard layers of guano during periods when the cave was uninhabited, so that succeeding groups of tool-makers settled on a cement-like floor of bird dung. About half of the 3,000 recovered specimens examined were discarded cores of a material that had to have been transported from some distance. This indicates that the inhabitants were engaged in tool manufacture. The Tabon fossils are considered to have come from a third group of inhabitants, who worked the cave between 22,000 and 20,000 BCE. An earlier cave level lies so far below the level containing cooking fire assemblages that it must represent Upper Pleistocene dates such as 45,000 or 50,000 years ago. Anthropologist Robert Fox, who directed the excavations, deduced that the Tabon Cave complex was a habitation of humans for a period of 40,000 years, from 50,000 to 9,000 years ago.