Heavy migration traffic and bad weather are a dangerous combination: Bird collisions in New York City

Bird–building collisions account for 365–988 million bird fatalities every year in the United States alone. Understanding conditions that heighten collision risk is critical to developing effective strategies for reducing this source of anthropogenic bird mortality. Meteorological factors and regional migration traffic may increase collision rates but also may be difficult to disentangle from other effects. We used 5 years of bird collision counts in New York City to examine the influence of nocturnal weather conditions and bird migration traffic rates on collisions with buildings during spring and fall. We found that seasonally unfavourable winds and conditions that impede visibility are important factors that increase the rates of bird–building collisions during both seasons. Specifically, northerly and westerly winds and low visibility in the spring and southerly and westerly winds and low cloud ceiling height in the fall are associated with higher collision risks. Generally, these weather variables associated most strongly with increased collisions when nocturnal bird migration traffic was high, with the exception of low visibility in spring, which was predicted to triple collision rates compared to high visibility, independent of bird migration traffic. Synthesis and applications: Although legislation to turn off unnecessary nocturnal lighting for the entirety of the migration seasons may be an ultimate goal, a proximate goal invaluable for reducing collisions will be predicting which nights will be of highest risk and using this information to determine when mitigation efforts could be most effective.