As many as 1 billion birds die each year from collisions with building glass -- from skyscrapers to homes. Birds simply can’t tell reflection from reality. The American Bird Conservancy is stepping up its attention to this danger. Check out:
This composite photo of birds killed at buildings in Baltimore is by Daniel Lebbin.

Category 3 of the basic requirements for being recognized as a Bird City is: Limiting or Removing Hazards to Birds. And one of those criteria asks that a "community provides easy-to-obtain information to property owners regarding protecting birds from window-strikes" -- that heartbreaking "thud" of a bird hitting one of our windows.

Daytime Collisions - Birds do not see glass the way humans do - they do not realize that glass is an obstacle. Birds often see reflections of nearby vegetation or the sky on the glass and unwittingly attempt to fly into this false landscape.　Especially dangerous are urban buildings with large windows within 40 feet of the ground, especially those surrounded by trees. Birds also collide with glass when windows align so that the bird can see into one window and out another window, creating a tunnel effect.　 And birds sometimes are attracted to indoor vegetation, such as large potted plants inside a corporate building atrium, visible from outside through windows, and attempt to fly to and land on this vegetation, colliding with the glass.

Night-time Collisions - Bright lights from buildings – especially tall ones -- at night confuse birds, especially during rainy or foggy weather.　 In Chicago and New York City, flocks of night-migrating songbirds numbering in the hundreds have been filmed circling in confusion around lighted skyscrapers and repeatedly colliding with lighted windows and building signage. Others circle these buildings until they drop, exhausted, onto the streets below. The next morning, the survivors may fall victim to urban scavengers or can't find their way out of the canyons of glass, concrete and steel.