Because birds do not understand glass as we do. Where we see reflections of trees, they simply see trees. Where we see a solid surface that separates indoors from out, they see open space.
The two main causes of collisions with glass are reflections and transparency:
Reflections
Birds are fooled by mirrored images of their surroundings, such as trees, shrubs, water and sky.
Because such reflections do not occur in nature (except on flat water), birds are unable to differentiate between illusion and reality.
Mirrored glass is especially dangerous, but all glass is reflective under certain light conditions.
Transparency
Many birds die trying to fly through glass or glass-like structures that offer an unobstructed view of vegetation or open sky on the other side. Examples include windows on opposite or adjacent sides of buildings, transparent balcony railings, glass windscreens, glass walkways and linkways, and bus shelters.
Birds seeking places to perch, eat or hide are also be attracted to houseplants, green walls and other indoor vegetation visible through windows.
IMPORTANT: Glass does not have to be clear to trick birds. Tinted glass, whether transparent or reflective, is equally dangerous. Neither is angling the glass an effective solution.
Other factors that influence the chances of birds colliding include:
Light pollution
Many birds migrate by night, using the moon and stars to navigate. Artificial lights disorient and confuse them, drawing them into the city. Some may collide with glass during the night, but most collisions occur after sunrise, when birds begin foraging and as they try to find their way out of the city.
Time of year
Collisions occur throughout the year, but peak during spring and fall migration, when many more birds pass through built-up areas.
Weather
Wind direction, temperature, air pressure and cloud cover affect migratory patterns and the altitude at which birds fly.
Other variables
The chances of colliding vary between species of birds, depending on how high and how fast they normally fly, as well as differences in vision. Migratory birds are more vulnerable because they are less likely to be familiar with human-built structures, and inexperienced juvenile birds are more likely to collide than mature ones.
Photo of Depot Boijmans Van Beuningen, Rotterdam: Hetty Kroese-Hoedeman