UAV‐assisted counts of cao vit gibbons 185


PLATE 1 Freeze-frame examples from the concurrent thermal and RGB UAV footage used in this study. (A) All five individuals in this group are discernible in the thermal video (three in A1 and two in A2). (B) At the same instance in the RGB video it is difficult to see the individuals with black fur, but one female can be seen clearly (B1) and another female is partially visible (B2).


from their characteristic yellow-blonde pelage (in contrast to the black fur of males, subadults and juveniles). We did not notice any obvious disturbances resulting from the pres- ence of the UAV except during our first trial of themethod. In this case, we believe that we approached the gibbons too closely (c. 40 m horizontal distance) and this seemed to cause the group to flee. The UAV approach detected some individuals that were


missed by the ground-based surveyors. The mean group size from the ground-based counts was 4.25 ± SD 1.50 compared to 6.00 ± SD 1.15 from theUAVfootage, a 41%increase. This is despite the fact that UAV observations were shorter in duration (on average 11 vs 81 min for direct observations). We estimated the difference in the group counts to be 1.79


(95% highest-density interval −1.46 to 4.83; Fig. 1), with an estimated 93% probability that this difference was greater than zero. This provides strong support for our hypothesis that the UAV-derived counts would miss fewer individuals and would therefore be higher. We also encountered two macaque groups during the


fieldwork (rhesus macaque Macaca mulatta and Assamese macaque Macaca assamensis) and collected video footage opportunistically. Counts of the macaque groups using the UAVwere substantially larger than those made by direct ob- servation (Table 1). We also used RGB footage to confirm species identity, which can prove difficult for field teams given the similarities between macaque species. We found evidence that aUAV-basedmethod of counting


primate group size wasmore effective than traditionalmeth- ods, producing higher counts and in a shorter amount of observation time. Critical to these counts was the use of a


FIG. 1 Posterior probability distribution of the difference in group size counts using a UAV-based approach vs direct observation. Positive differences mean that the UAV method counted more individuals. There was a 7.2% probability that the difference was less than zero and a 92.8% probability that it was greater than zero. The highest-density interval (HDI) is indicated by the horizontal black bar.


thermal camera, in which individuals of a group could be seen as hotspots against the cooler background of the sur- rounding forest. The UAV-derived count data have been incorporated into the 2021 population survey of the cao vit gibbon, leading to a more accurate estimate of the global population size than would have been possible using only the direct observation data. The UAV footage also helped to provide group composition data for the gibbons, which are also critical for models of population viability for the spe- cies (e.g.Wearn et al., 2021).Here, we used aUAV to support


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


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