Reactions of Sandhill Cranes Approaching a Marked Transmission Power Line

Abstract

Abstract Sandhill cranes Antigone canadensis, formerly Grus canadensis, are of widespread management focus, particularly where collisions with power lines are an important cause of mortality. Collision mitigation focuses on marking power lines to increase visibility, but collisions persist, perhaps because power line markers are not sufficiently visible in all conditions. Our objective was to compare reaction distances and reaction behaviors during daylight when power lines are presumably more visible, and during darkness when power lines are less visible. The power line we studied was fitted with glow-in-the-dark power line markers intended to increase nocturnal visibility. We found that during daylight, flocks generally avoided the power line by climbing gradually and passed above without making sudden evasive maneuvers. During darkness, flocks, particularly small flocks, were almost equally likely to make sudden evasive maneuvers as to climb gradually. Collision monitoring on the power line we studied conducted concurrent to our study indicated that 94% of collisions occurred during darkness, linking the behaviors we observed to actual mortality. Sandhill cranes also reacted at greater distances and with fewer sudden evasive maneuvers to the glow-in-the-dark-marked power line we studied than to nearby power lines without glowing markers evaluated in a prior study, suggesting that either glowing markers, smaller gaps between markers, or both, improved sandhill cranes' ability to perceive and react to the power line we studied. By correlating behavioral observations with mortality, our study indicates that proactive low-intensity behavioral observations might be useful surrogates to reactive high-intensity carcass searches in identifying high-risk spans. This approach may also be effective for other species.