Field validation of avian radar surveys: effects of differences in species and flock sizes on echoes

Abstract Radar is a powerful technology for surveys of avian movements. Validating the accuracy of radar detection is an essential step when establishing quantitative criteria for counting bird flocks. Previous studies on accuracy verification have focused on trajectory data after tracking echoes. However, as tracking algorithms differ among researchers, so radar detection performance has not been accurately validated. This study clarifies the positional and biological factors that influence the probability of detection (POD) and area (pixel size) of echoes on X-band marine radar. As positional factors, distance was found to have a negative effect on POD and pixel size, while elevation angle was a positive effect on POD. We confirmed that biological factors included species differences (body weight, wingspan, and soaring behavior) and waterfowl flock size. Body weight and flock size positively affected POD and pixel size. In predicting detection performance, the survey distance required to achieve 50% POD was 750 m or less for Grey-faced Buzzard Butastur indicus (0.4 kg), the lightest target species, but up to 1800 m for a pair of Bewick’s Swan Cygnus (columbianus) bewickii . This study contributes to establishing more relevant criteria when setting the range of radar surveys and tracking algorithms of echoes.