Monitoring roosting waterbirds: The use of drones to overcome the challenge of hidden individuals in blind zones on intertidal flats

Several wildlife censusing techniques depend on direct visual observations for detection and enumeration of populations. In terrain with challenging topography, awareness of areas where animals may not be visible is essential for reliable monitoring and to avoid surveyor biases caused by choice of censusing approach, which may fail to detect certain individuals. Monitoring in remote and inaccessible waterbird habitats often presents such risks, since traditional ground-based counts are performed from a limited number of vantage points. This study investigated the occurrence and extent of ‘blind zones’, where waterbirds could not be seen, when surveying high-tide roosts with ground counts from vantage points on adjacent seawalls in the Danish Wadden Sea. We mapped blind zones in geographical information systems using visibility simulations and validated our results with field measurements. We also investigated the potential use of unmanned aircraft systems (UAS) to survey the number and spatial positions of otherwise hidden individuals in the predicted blind zones. Substantial blind zones were detected in sections with elevated foreland, where birds up to 600 m out on the intertidal flats could not be seen when observing from the adjacent seawall. In these problematic zones, we predicted that 51–61% of the bird aggregations would not be visible during a traditional ground survey. We examined whether certain species are less likely to be visible depending on the species’ height
when standing and general habitat preferences. We found that high percentages of small- and medium-sized waders (Dunlin Calidris alpina, European Golden Plover Pluvialis apricaria and Northern Lapwing Vanellus vanellus) and Common Gull Larus canus frequently roosted in blind zones. Our study suggests that there could be substantial risks of underestimation of roosting waterbirds during ground-based counts. Furthermore, surveys with UAS of the blind zones detected otherwise unnoticed individuals, and thus showed a compelling potential for UAS as a complementary research tool for waterbird censusing.