Collision Avoidance by Wintering Bald Eagles Crossing a Transmission Line

Abstract

Collision with electric lines is a global conservation concern for many birds but raptor species are generally thought to have a low risk of line collision. Collision risk at electric lines is not well understood for Bald Eagles (Haliaeetus leucocephalus), with only incidental reports of collisions in the literature. Line collisions reported for Bald Eagles predominantly occur where lines intersect movement corridors around foraging, roosting, or nesting areas. During five winters from 2014 to 2018, we monitored 602 Bald Eagle crossings of a 230/500-kV transmission line at the Delaware River to determine collision risk. Eagles successfully crossed the line by flying predominantly below the wire zone. There was no difference in flight heights for immature or adult eagles crossing the line. Wire-marking protocols within the electric industry are primarily focused on marking the top plane of wires to alert birds flying above the wires. Additional development of wire-marking options for energized conductors could further reduce collision risk for eagles and other bird species flying below wires. Overall, collision risk appears low for Bald Eagles making localized movements across transmission lines.