Combining distance sampling and triangulation to estimate density of elusive rainforest vertebrates

Knowledge of the density of endangered or game species is important for conservation and management. Distance sampling and triangulation have been used in acoustic population estimates of primates and birds. We tested a novel combination of these two methodologies to obtain a reliable density estimation of the New Caledonian imperial-pigeon Ducula goliath, which is both a traditional game and an emblematic endemic near-threatened species. We conducted 100 acoustic point counts in the Parc Provincial des Grandes Fougères (45 km²) on mainland New Caledonia during which two observers simultaneously located calling imperial-pigeons by taking bearings during a 5-min interval. We then used the locations obtained by triangulation for density estimation by distance sampling. At the same time, we assessed the proportion of imperial-pigeons that called during the sampling interval. This allowed us to obtain a corrected population density estimate that included the entire population and not just the calling birds. The number of imperial-pigeons heard per listening point varied with time of day and between observers. In contrast, distance sampling combined with triangulation provided similar estimates for the two observers. During the dry season, with a proportion of calling individuals of 60 %, the method estimated a corrected density of 35 imperial-pigeons/km² (24–50 CI) and a population size of 1502 (1044–2159). We developed a cheap and simple method to provide reliable density estimates of a wide range of species employing their vocalisations in tropical forests and that can be used by scientists and well-trained field workers alike.