ABSTRACT Collisions with tall anthropogenic structures (power lines, wind turbines, communication towers, and buildings) are a regular cause of mortality for nocturnal avian migrants. To better understand relationships between weather and migrant collision risk, we compared nocturnal hourly mean barometric pressure, temperature, relative humidity, wind direction, and wind speed to avian passage rates (targets/km/h) and flight heights during 1 year of spring and fall migrations near a recently reconstructed transmission line crossing Kittatinny Ridge, New Jersey, using 2 marine radars. We found lower spring flight heights associated with decreasing barometric pressures, temperatures, and relative humidity across all wind directions. Spring flight heights showed the strongest decreases with increasing wind speeds for headwinds and crosswinds while flight heights slightly increased with stronger tailwinds. Spring passage rates increased with lower relative humidity and higher temperatures. With increasing wind speeds, spring passage rates declined faster with headwinds and crosswinds, and only slightly with tailwinds. We found lower fall flight heights with decreasing temperatures and relative humidity, but the relationships varied by wind direction. The lowest flight heights occurred under tailwind and crosswind conditions. Increasing fall passage rates were associated with increasing barometric pressures and when wind speeds were lowest. Fall passages rates declined with higher wind speeds but were more gradual under tailwinds and crosswinds when compared to headwinds. Overall, 2–4% of birds migrated under conditions that would have placed them at risk of collision with the transmission lines. Similar studies conducted along migratory pathways could be used to predict nights where birds exhibit low flight heights around other obstacles along their migration path. These predictions could improve detection rates for carcass searches documenting relatively rare collision events. For sites with topographic features likely to concentrate nocturnal migrants, radar studies could be conducted before siting towers so that impacts to migratory birds could be avoided at these sites.

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.

Avian collisions with overhead power lines are of conservation concern, particularly in migration corridors. We studied potential collisions where an existing power line supported by towers 20-25 m tall was replaced by the Susquehanna-Roseland line (S-R line), a new line with towers 55-60 m tall. The S-R line crosses Kittatinny Ridge, a corridor for raptors migrating south through New Jersey and Pennsylvania, USA. We hypothesized that the S-R line, which on Kittatinny Ridge includes markers designed to increase its visibility to birds, would cause migrating raptors to react in 1 of 3 ways: (1) to not alter flight elevation, but to pass safely through the S-R wire zone; (2) to not alter flight elevation, and to not pass safely through the wire zone, leading to collisions; or (3) to alter flight elevation and to pass safely above or below the S-R wire zone. To evaluate these hypotheses, we recorded the flight elevations of migrating raptors in 2013 before construction of the S-R line and in 2014 postconstruction. Preconstruction, we recorded 3,698 raptor crossings. Most raptors (72%) crossed above the anticipated S-R wire zone. Some (24%) passed through the anticipated S-R wire zone, and a few (4%) passed below the anticipated S-R wire zone. Postconstruction, we recorded 4,482 crossings. Most raptors (92%) crossed above the S-R wire zone. A few passed through (5%) or below (3%) the S-R wire zone. Postconstruction, raptors responded to the new line by flying higher than they had when traversing the previous line. We did not observe any collisions. Altered flight elevations and the absence of observed collisions supported hypothesis 3. If similar patterns occur at other lines that cross diurnal migration corridors along ridges, then future monitoring may be better focused on potentially riskier settings, such as areas where migrating birds do not have deflected winds to assist with gaining elevation.