184 O. R. Wearn et al.


TABLE 1 Primate group size counts made by direct observation and derived from UAV video footage in the Cao Vit Gibbon Species and Habitat Conservation Area, Vietnam.


Direct observation Species


Cao vit gibbon Nomascus nasutus


Cao vit gibbon


Rhesus macaque Macaca mulatta


Assamese macaque Macaca assamensis


Cao vit gibbon Cao vit gibbon Total


Date, time


27 Oct. 2021, 10.00


28 Oct. 2021, 9.32


4 Nov. 2021, 13.05


6 Nov. 2011, 8.46


9 Nov. 2021, 9.22


10 Nov. 2021, 6.24


Duration (min)1


182 28 8 3


50 65


336


Count Composition2 5


2 4 3 5 5


24


2 female, 3 black 1 female, 1 black Unknown Unknown


2 females, 2 black, 1 small infant


2 females, 3 black UAV observation


Duration (min)1


5


14 10 8 6


17 60


1Total duration of each observation. 2Black individuals are either males, subadults or juveniles; these age classes cannot be distinguished easily in the field.


Despite the importance of the average group size param-


eter, obtaining an accurate estimate has proved difficult in practice (Trinh Dinh et al., 2018; Ma et al., 2020). Group size counts are made typically at distances of 30–500 m, with individual gibbons often passing in and out of view as they move through the dense vegetation. Observers must attempt to track multiple individuals as they move or they must wait for all gibbons to enter a feeding tree with a relatively open canopy (which happens infrequently). As a result, gibbon group counts are often incomplete and average group sizes are underestimated. New technologies are beginning to overcome some of


the difficulties associated with surveying gibbons and other cryptic primates, including the use of camera traps and acoustic recorders (Piel et al., 2021). In this study, we propose a new method for counting gibbon group size using unoccupied aerial vehicles (UAVs) and report on their use during a population survey of the cao vit gibbon. Our hypothesis was that group size counts us- ing UAVs would miss fewer individuals, and would there- fore be higher than those from traditional ground- based observations. During 26 October–10 November 2021, we surveyed the


cao vit gibbon population from 30 mountain-top survey posts across the range of the species in Vietnam (Cao Vit Gibbon Species and Habitat Conservation Area) and China (Bangliang National Nature Reserve), with methods closely following those of previous surveys (e.g. Ma et al., 2020). Each post was monitored by an observation team of 2–3 people for several (range 1–9) consecutive mornings (4.00–12.00). Independent from the observation teams, we also had a roving drone team, comprising a pilot and as- sistant, with a portable UAV (DJI Mavic 2 Enterprise


Advanced, Shenzhen, China) and five batteries (each pro- viding a maximum of 31 min of flight time). The UAV was equipped with a 640 × 512 pixel thermal camera and a 48 megapixel standard (RGB) camera. For each survey day, the drone team chose a survey post that was thought to have a high chance of detecting gibbons that were close by and/or observable (based on gibbon records from previ- ous days and long-term monitoring data). If the observation teams sighted or heard gibbons, the drone team flew the UAV manually to the approximate location and attempted to obtain video footage. Flight heights were 30–120mabove the ground surface, camera angles were not standardized and line of sight with the UAV was maintained. We did not fly speculative missions to search for gibbon groups, to reduce any potential disturbance from the UAV during the population survey. Author ORW later reviewed the video footage from the


UAV to determine group sizes. We then compared these group sizes statistically to those made independently by the observation teams, using Bayesian paired sample t tests (Kruschke & Meredith, 2021). Across 10 survey days, 6 had suitable weather for flying.


We obtained video footage (simultaneous thermal and RGB) of four gibbon groups and could make group size counts in each case (Table 1), with individual gibbons evi- dent as bright spots in the videos (Plate 1; Supplementary Video 1). We found it difficult to identify thermal bright spots conclusively as gibbons if individuals were stationary, but their identification became clear once they moved be- cause of their unique locomotory style and relatively long forelimb length. We found the RGB footage to be more use- ful than the thermal footage for determining group compos- ition, particularly for identifying the number of females


Oryx, 2024, 58(2), 183–186 © The Author(s), 2023. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605323000017


Count Composition2 5


5


22 13 7 7


59


1 female, 3 black, 1 unknown


2 female, 3 black Unknown Unknown


2 females, 4 black, 1 small infant


2 females, 2 black, 2 unknown, 1 infant


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