Detection Patterns and Population Monitoring of Sympatric Hornbills in the Ebo Forest, Cameroon

Forest hornbills are major seed dispersers, but dense vegetation, canopy use and species specific vocal behaviour make their populations difficult to monitor. These constraints are especially important in the Ebo Forest, where the Vulnerable Yellow-casqued Hornbill Ceratogymna elata occurs with a diverse sympatric hornbill assemblage under growing hunting and land-use pressure. We evaluated detection patterns and established a field-observation baseline for population monitoring of C. elata and co-occurring hornbills. Hornbills were recorded using 15-minute point counts at 98 survey stations in 2025. We quantified aural, visual, and combined detections, comparing the estimated detection distances and observed heights among common species. Additionally, retrospective aural- and visual-inclusive data subsets were analyzed to demonstrate the extent of survey coverage retained by each individual information channel. Hornbills occurred at 85 stations, yielding 186 species-by-station events, 413 individuals and 10 species. Aural-only detections comprised 58.6% of events, visual-only detections 28.5%, and simultaneous aural-visual detections 12.9%. A visualinclusive subset retained only 41.4% of events, eight species and 54 occupied stations, whereas an auralinclusive subset retained 71.5% of events, all 10 species and 69 stations. Detection distance differed among six common species (Kruskal-Wallis H = 14.56, P = 0.012). Observed height differed strongly among species (H = 54.26, P < 0.001), with large Ceratogymna and Bycanistes recorded higher than smaller hornbills. Population monitoring in the Ebo Forest should combine aural and visual records, repeated visits, observer training and acoustic validation. The present results provide a detection baseline and identify the observation biases that future, population size estimate, occupancy, density and community-based monitoring programmes must address.

Keywords: Acoustic Monitoring, Avian Detectability, Bucerotidae, Point-Count Surveys, Tropical Forest Monitoring