BIODIVERSITAS ISSN: 1412-033X Volume 21, Number 5, May 2020 E-ISSN: 2085-4722 Pages: 2088-2096 DOI: 10.13057/biodiv/d210536 Nesting behavior of reintroduced Bornean Orangutan in Bukit Batikap Conservation Forest, Central Kalimantan, Indonesia IKE NURJUITA NAYASILANA 1,4, , SRI SUCI UTAMI ATMOKO 2 , AHMAT SUYOKO 3 , SUWARNO HADISUSANTO 4 1 Program of Forest Management, Faculty of Agriculture, Universitas Sebelas Maret. Jl. Ir. Sutami No. 36A, Surakarta 57126, Central Java, Indonesia. Tel.: +62-271-637457,  email: nayasilana@gmail.com 2 Primate Research Center, and Faculty of Biology, Universitas Nasional. Jl. Sawo Manila, Pasar Minggu, Jakarta Selatan 12520, Jakarta, Indonesia 3 The Borneo Orangutan Survival Foundation. Jl. Kumbang No. 31, Bogor 16128, West Java, Indonesia 4 Faculty of Biologi, Universitas Gdjah Mada. Jl. Teknika Selatan, Sekip Utara, Sleman 55281, Yogyakarta, Indonesia Manuscript received: 6 February 2020. Revision accepted: 20 April 2020. Abstract. Nayasilana IN, Atmoko SS, Suyoko A, Hadisusanto S. 2020. Nesting behavior of reintroduced Bornean Orangutan in Bukit Batikap Conservation Forest, Central Kalimantan, Indonesia. Biodiversitas 21: 2088-2096. Great apes such as orangutans build a nest to rest in every night. Orangutan’s nest making skills correlate with their adaptability in a new habitat. Here we analyzed the nest skill on the reintroduced orangutan. Skills process and thought training there is a back to remembers in forest school and pre-release islands in the rehabilitation center. The study aimed to observe the skill adaptation of reintroduced orangutans (semi-wild and ex-rehabilitate) for nest building and the characteristic of the tree chosen for their nests in a new habitat. The study was 16 individuals (8 semi-wild and 8 ex-rehabilitate) from February 2012 to November 2014 at the release site of Bukit Batikap Conservation Forest, Central Kalimantan. The method used in the study was instantaneous focal animal sampling, which follows the behavior of each orangutan from the morning nest to the night nest. Non-parametric statistics test (Kruskal-Wallis) was used to nesting behavior, Canonical Correspondence Analysis (CCA) is used to determine the relationship between nesting behavior and nesting tree species, and ArcMap GIS 10.3 to find out the distribution of nest trees in the research location. The result of the study revealed that reintroduced orangutan prefers to build a nest to rest at night rather than day nest. Semi-wild orangutans prefer tree species with dense canopy and branching, while ex-rehabilitation orangutans pay less attention to this. Mostly, reintroduced Bornean orangutan created new nests rather than reusing and/or repairing old nests. Species of Syzygium sp. Lithocarpus sp. and Xanthophyllum sp. are a preferred species of the nest tree, the nest position of 2 (nests are constructed on the main side-branch) and 3 (the crown of the tree) is dominance. Orangutan prefer height tree species between 26-30m for day nests, are selectively determined as part of the adaptation process. However, each individual will choose the nest type species of tree, positions, and height according to their characteristics of the nest tree. Keywords: Characteristic-tree, nest, orangutan-reintroduction, species-tree INTRODUCTION Orangutans are arboreal animals that are sometimes difficult to detect in the forest. Orangutans build nests every night to sleep and sometimes also make nests during the day (Prasetyo et al. 2009). Nesting behavior will be carried out at the end of each activity, such behaviors include: breaking of branches, gathering branches and twigs to form as a body protector at rest. In general, the nest is made according to its completeness starting from the base, pillow, blanket or cover (roof). Orangutans will also choose tree species and tree height before making a nest. The selection of tree species is related to the selection of leaves, branches, and twigs, which are then made of foundations and nodes (Prasetyo 2006). Several parameters are needed when researching nests, namely height, diameter and type of nest tree; the height and position of the nest; nest conditions used (built, rebuilt or reused); use of feed trees as nest trees; and the time of building and use of nests (van Schaik et al. 1995; Buij et al. 2003). Whereas the position of the nest is divided into four positions, namely the nest which is at the base of the main branch and attached to the main stem; nest of the middle or end position of a tree branch; the nest is at the top (top) of the main tree; and nests built of two or more trees (van Schaik 2006; Prasetyo et al. 2009). Orangutans will like the nest with a position not far from the canopy, but not in a position that is too open. This is to avoid attacks by night predators (Sugardjito 1986; Ancrenaz et al. 2004). Young orangutans use their afternoon nests repeatedly (Rijksen 1978; Maple 1980), while the orangutans that have been independent will make new nests and are generally around the last feed tree or in certain cases in the last feed tree (Sugardjito 1986, Prasetyo 2006). These orangutans generally built new nests, especially when building night nests (Rijksen 1978; Galdikas 1986; van Schaik 2006). MacKinnon (1978) and van Schaik (2006) added that the selection of new nest tree species was believed to be strong and comfortable, especially those with many branches and had broad and soft leaves. The ability and skills of nesting will develop with age. In general, at the age of 3 (three) years orangutans can do these skills. Their skills will continue to grow with age (van Schaik 2006; van Noordwijk 2009). Nest-making skills are also inseparable from the presence of tree branches as a foundation, and the twigs needed by young