By Aidan LewisBBC News, Washington
Every year, hundreds of millions of birds are killed or injured when they fly into windows. Volunteers who document the collisions are now calling for architects and landlords to make their buildings more bird friendly to reduce the number of deaths.
Sometimes birds see plants or empty spaces beyond the windows - sometimes they just see the reflection of the sky or trees, but not the glass itself.
Most tend to cruise at 20-30mph (32-48kph) - if they hit a window at that speed the impact is usually fatal as their beak is jammed back into the brain.
Although collisions can happen anywhere, they are most common in cities where big glass buildings proliferate.
Local birds seem to learn where they can fly safely but migratory songbirds such as warblers, thrushes and sparrows have a particular problem identifying glass.
They usually fly by night when they are less visible to predators and use the stars to navigate - but they appear to get confused by the illumination of towns and cities below.