To conserve biodiversity scientists monitor wildlife populations and their habitats. Current methods have constraints such as the costs of ground or aerial surveys, limited resolution of freely-available satellite images, and expensive high resolution satellite images. Recently researchers started to use unmanned aerial vehicles (aka UAVs or drones) for wildlife and habitat monitoring. Here we tested whether we could detect nests of the critically endangered Sumatran orang-utan on imagery acquired from camera mounted drone to determine distribution and density. Our results show that the distribution of nests compares well between aerial and ground based surveys and that relative density (nest/km) shows a significant correlation between these two survey types. The results also indicate that both methods can be used to detect significant differences in relative density between previously degraded reforested and enriched areas. We conclude that orang-utan nest surveys from drones are a promising survey method to determine distribution and (relative) density of this and perhaps other ape species.

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This study investigated the reproductive parameters of free-ranging rehabilitant female orangutans. We aimed to assess the factors that influence these parameters and provide information that could assist with the management of orangutan reintroduction programs. We analyzed the birth records of free-ranging female rehabilitants at Bukit Lawang, Bukit Tigapuluh, Sepilok, Camp Leakey, Kaja Island, Sungai Wain, and Meratus and compared them with reproductive parameters reported in wild and zoo populations. Females' ages at first birth were 10.6-14.7 years, significantly earlier than those of wild and zoo orangutans. Computed inter-birth intervals (IBIs) calculated by the Kaplan-Meier method were 65.1-90.1 months; the values for Camp Leakey and Bukit Lawang rehabilitants were significantly shorter than those reported for wild Sumatran orangutans. Infant mortality rates were 18-61%; the values for Bukit Lawang and Sepilok were significantly higher than those reported for wild Sumatran and zoo orangutans. In rehabilitants, young ages at first birth and shorter IBIs may result from the high energy intake enabled by provisioning, although the possibility exists that they reflect underestimations of age on arrival at rehabilitation centers. The observed high infant mortality rate may reflect poor mothering skills due to human rearing and/or increased disease transmission. This study demonstrates that accelerated reproductive rates (younger age at first birth and shorter IBI) are common in provisioned rehabilitant females on both Sumatra and Borneo.