Steel and Glass in the City of Chicago

This past Saturday, I wrapped up my first season as a volunteer for Chicago Bird Collision Monitors, a program of the Chicago Audubon Society. Before I began this patrol of the city streets, I would occasionally see dead birds on sidewalks downtown and thought they must have been poisoned by the City’s rat abatement project. I knew birds collided with buildings, but I did not realize how common an occurrence this really is.

Ovenbird in a Net
Ovenbird, downtown Chicago, photo by L.J. Bailey

Why Chicago Matters: Forty percent of all North American migratory birds travel on a route known as the Mississippi Flyway, a sort of superhighway for birds, and part of the route passes right over the City of Chicago. The sheer scale of the migration through the city is almost unimaginable: eight million individual birds from 250 distinct species. Unfortunately, this makes our beautiful skyline notorious for killing more birds than many other skylines in North America. One Streets and Sanitation worker I spoke with estimated that they find tens of thousands of dead birds each migration season in one part of the downtown area alone.

Why Building Glass and Lighting Matter: Most migratory birds migrate at night because they rely on the sight of the Earth’s magnetic field lines, the sun and other stars, and the moon. Night lighting used in dense urban areas obscures these navigational aides and confuses the birds. Birds are instinctively attracted to the lights and may circle the air for hours around beams of light until striking a building or dropping from the sky in exhaustion.

Other birds seeking food, water, and shelter on their long journeys spy plants, fountains and other enticing “natural” displays in glassed-in lobbies and atriums and attempt to fly to them, striking glass. The transparent and reflective properties of glass create the illusion of open space and inviting habitats for the birds. Even though a bird may be taking off from the ground, it can reach a deadly speed of thirty miles per hour or more relatively soon after takeoff.

Someone recently suggested to me that birds colliding with glass was probably the best example of Darwin’s “survival of the fittest” at work. In my next post, I will explain why this is not a scientifically valid conclusion.

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