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.

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