Glass kills large numbers of birds in American cities, and the reason is perceptual rather than behavioral. Birds do not see glass as a surface, so they fly toward what it reflects.
Reflection creates false habitat
A window mirroring nearby trees or sky presents an image a bird reads as open flight space. Nothing in the visual field signals an obstacle.
Reflectivity increases with the difference in light between outside and inside, so a shaded interior behind bright exterior light produces the strongest mirror effect.
This is why the ground floors of buildings, surrounded by plantings and set below overhangs, generate collisions out of proportion to their glass area.
Transparency creates the other trap
Where a bird can see through a building, as with glass corridors, corner lobbies or clear railings, it perceives an actual route rather than a reflection.
Interior plantings visible through glass make this worse by adding a destination. The bird is flying toward vegetation that genuinely exists, just not reachable.
Night lighting compounds both problems during migration, drawing birds into lit urban areas where they encounter far more glass than they otherwise would.
Spacing matters more than pattern
Effective treatments break up the reflective surface with markings dense enough that no gap looks passable. What counts is the size of the untreated space between marks.
Widely spaced decals do very little, because a bird will attempt the opening between them. Uniform patterns across the whole pane work far better than a single silhouette.
Markings need to be on the outer surface. Applied inside, they sit behind the reflection they are meant to interrupt.
Design choices at the drawing stage are cheapest
Fritted glass, etched patterns and external shading are integrated into the pane or the facade during construction and require no maintenance afterward.
Retrofitting an existing tower means film or exterior cords, which cost more per window and must be renewed. The decision point that matters is architectural.
Several US cities have adopted bird-safe requirements for new construction, generally applying to lower stories and to glass adjacent to vegetation or water.
Lights-out programs address the other half
Reducing nighttime lighting during migration seasons lowers the number of birds drawn into downtown areas in the first place.
These programs are voluntary in most places and rely on building managers switching off unoccupied floors, which also lowers energy costs.
Neither approach substitutes for the other. Lighting determines how many birds are present; glass treatment determines what happens to the ones that arrive.