Glass is Air

Birds don't see glass.  People do - at least most of the time. At some point we've all walked into a glass door or window but nevertheless, we understand glass. We have expectations of where it will be, even when it is too clear to see. Other species do not. Clear glass is air. Birds fly right into it.
 

A common building lobby with clear glass, trees and a blue sky beckoning in the distance.

Clear glass pedestrian bridges two blocks from the lake shore.

Glass wall between two sections of a building - even people have a hard time knowing it's there.

Windows are Trees

When birds look at these windows, what they see are the trees.
 

Street lighting on this block is so bright the trees from across the street are reflected at night.

A typical park area nestled between buildings; the trees reflect off windows in every direction.

Birds flying along the row of trees will hit the building head-on.

Buildings are Sky

Birds are completely fooled by the reflection of sky and clouds in windows and especially tall buildings covered with curtains of uninterrupted glass. This type of hazard is highly dependent upon the weather conditions. A building may be harmless most of the day yet pose a significant hazard when the sun reaches a certain angle.
 

The reflection of sky is so strong here that the window washers reported birds flying into the buildings all day long.

While the photographer was watching this building an American Crow ran into it at about the 15th story. The flat glass mirrors the moving clouds perfectly.

A typical corporate building with expansive windows reflecting the trees and sky.