Mortality Records Guide UAV-Based Assessment of Power-Line Conspicuity at a Black-Necked Crane Breeding Site

Overhead power lines increasingly intersect alpine wetland and grassland habitats on the Qinghai-Tibet Plateau, but variation in wire conspicuity within collision-associated areas remains poorly quantified. We combined GPS-GSM tracking and field verification to document infrastructure-associated mortality and separately used standardized UAV RGB images to characterize power-line conspicuity at the Luanhaizi Wetland, Qinghai Province. From 2020 to November 2025, we monitored 40 Black-necked Cranes (Grus nigricollis); nine were alive at the data cutoff, 23 were confirmed dead, and eight were lost to follow-up. Among the confirmed deaths, nine were classified from field evidence as wire-mesh fence entanglements and 14 as power-line collisions. All 14 field-inferred collision deaths involved 110 kV lines in this monitored sample. Accordingly, the UAV assessment focused on a selected section within the documented 110 kV collision-associated area; this site-selection criterion does not demonstrate that 110 kV lines are intrinsically more hazardous than other local voltage classes. We analyzed 640 UAV images and calculated an image-based visibility index (VI) combining normalized RGB target-background contrast and edge strength. VI describes relative wire conspicuity in standardized photographs rather than Black-necked Crane visual perception or collision probability. Blocked non-parametric tests showed that VI was lower under overcast than clear conditions (p < 0.001) and varied with observation distance and relative UAV height (both p < 0.001), whereas viewing angle was not significant (p = 0.065) and time of day showed a weak effect (p = 0.030). VI rankings remained reasonably stable under alternative component weightings. This two-stage approach uses documented mortality to identify candidate sections for image-based assessment but does not test whether low VI predicts collisions. It can support targeted marker trials that require independent evaluation through standardized collision monitoring.