A simple and low-cost method to estimate spatial positions of shorebirds: the Telescope-Mounted Angulator

Estimating the spatial position of birds in open habitats like intertidal mudflats is important for many studies, for example, detailed density estimates or linking predation pressure to resource availability. To date, several methods have been used to estimate the positions of birds, including density counts in predetermined plots, range finders, photography, and tracking individuals tagged with GPS-equipped transmitters, and each method has advantages and shortcomings. Counts in premarked plots are possible over relatively long distances, but small-scale information is lost due to within-plot averaging. Other methods accurately determine the position of individuals, but can only be used at relatively short distances or involve capturing birds. We describe a simple and low-cost method to estimate the spatial position of individual birds in open habitats using a telescope-mounted instrument that measures the scope's viewing angle. Using this Telescope-Mounted Angulator (TMA), the distance to focal birds can be calculated by simple trigonometry, requiring only the viewing angle and mounting height of the telescope. Laboratory tests revealed that the TMA was most accurate when calibrated for individual observers. Field experiments performed on a 4-m high observation platform showed that the TMA can estimate the position of shorebirds with an accuracy of 18 to 36 m up to a distance of 500 m. By also including the direction, determined with a compass, the spatial position of birds can be reliably estimated. The TMA can be a valuable tool for estimating the spatial position of animals in various flat landscapes, providing detailed measurements in a relatively short period of time.