Orang-utan nest surveys: the devil is in the details

Nest surveys are widely employed to assess the population density of orang-utans (Pongo spp.) and evaluate alternative management scenarios relevant to the protection of these threatened great apes. However, this method is less accurate and prone to much greater error than is generally acknowledged. Here we highlight the limitations of orang-utan nest surveys, discuss the risks of ignoring these limitations, and note conditions under which standard nest survey methods are appropriate.