Monitoring wild cervid populations have become a priority for management. However, accurate and reliable estimates of densities are difficult to achieve since they may be affected by environmental variation, species behaviour or observational issues. Therefore, to obtain unbiased estimates of densities it is necessary to adopt sampling methods that quantify the probability to detect the target species. In this study, we compare the results of roe deer sampling based on distance detection performed by two techniques: surveys on foot in the evening and nocturnal surveys by car. Estimates of roe deer population densities were conducted in Sierra de Guadarrama (Madrid, Spain). Distance sampling was conducted along tracks in 10 pine forests in October. Observations from the surveys done on foot were better fitted with detection functions, although this technique required more days and more observers for its realization, hence increasing field effort. Nocturnal surveys by car were also a proper technique and decreased distance sampling costs, since only three people were needed for 6 days to carry them out. However, observations obtained with this technique showed an imbalance in the detection function in the first few metres. This model was limited by the small number of roe deer observed in or near the line of progression. This is a handicap because functions used by the Distance software assume that the highest probability of detecting specimens is in the line of progression, causing an imbalance in the detection function at zero distance. To compensate for this, data were lefttruncated at 20 m. Therefore, when it is necessary to estimate absolute densities of roe deer populations, nocturnal distance sampling by car seems to be the most appropriate method due to its low cost, yet the influence of the vehicle on the distribution of roe deer and, therefore, on the estimated density, must be taken into account when carrying out such studies.
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