Cities host a variety bird species (listen to Haikubox Northern Flicker recordings made this week in Manhattan and Chicago) and millions of birds fly by them during spring and fall migration.
Those routes can prove deadly when birds mistake reflective windows for open sky, as they did in Chicago last October. Warm weather and southerly winds had stalled migration until a cold front provided the first opportunity for hundreds of thousands of birds to resume their journey. Sadly, many did not do so safely, and almost 1,000 birds collided with one lakeside building.
A 2024 paper looked at the environmental factors that affected bird-building collisions in New York City. The authors used community science data to determine that higher collision risks were associated with unfavorable headwinds and low visibility or cloud ceiling height, and were worst when nocturnal bird migration traffic was high. This leads birds to fly at lower altitudes in search of suitable stopover locations where they can wait out bad weather.