Advances in Animal and Veterinary Sciences August 2023 | Volume 11 | Issue 8 | Page 1332 INTRODUCTION T he Kalimantan orangutan (Pongo pygmaeus) is one of the Asian great apes whose distribution is limit- ed to Kalimantan and some parts of Malaysia (Sunder- land-Groves et al., 2021; Rocque et al., 2022). Te Interna- tional Union for Conservation of Nature (IUCN) classifed it as Critically Endangered. Simultaneously, it is listed as Appendix I (trade prohibited) in the Convention on Inter- national Trade in Endangered Species of Wild Fauna and Flora (CITES) (Ancrenaz et al., 2018). Te conservation status was given to P. pygmaeus based on habitat loss and population decline due to land conversion and confict with humans ( Yuliani et al., 2023). Te cause of the confict is often due to orangutans trespassing into community gardens and eating the produce. Nursing or- angutans are shot and often killed, and babies are sold on the black market for pets (Davis et al., 2013; Gnuse, 2022; Sherman et al., 2020). Research Article Abstract | Tis research investigated nesting behavior in Bornean orangutans (Pongo pygmaeus) as a suitable release subject at the Sekolah Hutan Tembak Lestari forest school, Sintang, West Kalimantan, Indonesia. Tis study aims to evaluate two orangutan release candidates through nest-building skills. Tese two orangutans were observed daily be- tween 04:30 and 06:30 pm from September to November 2019 using continuous focal animal and all-occurrence sam- pling methods. Nest-building skills were assessed using the Likert scale. Moni (male orangutan) was always observed to nest using used nests. Meanwhile, Terra (female orangutan) was frequently observed building new nests, although she was caught sleeping on the forest foor several times. During observations, it was found that they prefer nest trees with a circumference of more than 1 m with a reasonably dense canopy refers preferred trees that serve as nesting sites, those are bambang tree (Lithocarpus sp.) with a tree circumference of 3 m and kempilik tree (Quercus sp.) 2.46 m and often reuse or rebuild old nests. Our results showed that both orangutans had moderate predicate nest-building skills, with a percentage of 68.06% for Moni and 63.89% for Terra. Nonparametric Mann-Whitney U statistical tests, (α = 0.05; p = 0.415) showed that the nest-building ability between the two individuals is not signifcantly diferent. With nest-building ability not signifcantly diferent between the two orangutans with a moderate predicate, the two orangutans can be released together. Keywords | Forest school, Nest building, Pongo pygmaeus, Rehabilitant orangutan, Release criteria LUTHFIRALDA SJAHFIRDI 1* , ZULFI ATSIIL SUANDHY 1 , TATANG MITRA SETIA 2 Nesting Behavior of Kalimantan Orangutan Pongo pygmaeus as a Release Subject at Forest School Sekolah Hutan Tembak Lestari, Sintang, West Kalimantan, Indonesia Received | May 11, 2023; Accepted | June 01, 2023; Published | June 15, 2023 *Correspondence | Luthfralda Sjahfrdi, Department of Biology, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok, Indonesia; Email: luthfralda@sci.ui.ac.id Citation | Sjahfrdi L, Suandhy ZA, Setia TM (2023). Nesting behavior of kalimantan orangutan pongo pygmaeus as a release subject at forest school sekolah hutan tembak lestari, sintang, west kalimantan, indonesia. Adv. Anim. Vet. Sci. 11(8): 1332-1337. DOI | https://doi.org/10.17582/journal.aavs/2023/11.8.1332.1337 ISSN (Online) | 2307-8316 Copyright: 2023 by the authors. Licensee ResearchersLinks Ltd, England, UK. Tis article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY ) license (https://creativecommons. org/licenses/by/4.0/). 1 Department of Biology, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok, Indonesia; 2 Faculty of Biology, Universitas Nasional, Jakarta, Indonesia.