Birds don't understand glass. It's estimated that a billion birds die each year from crashes into glass.

Birds don’t understand glass. It’s estimated that a billion birds die each year from crashes into glass.

1 billion birds die every year by flying into glass they can’t see. Companies are developing bird-safe glass that is coated with a patterned, UV-reflective glaze, and cities around the US are taking action. One New York City convention center was renovated with bird-safe glass, and it reduced collisions by 90%.

Why Birds Can’t See Glass?
1. Birds Don’t Understand Reflections.
2. Birds Don’t Know To Look for Glass.
3. Birds See Color More Intensely.
4. Birds Have Poor Depth Perception.
5. Birds Aren’t Used to Vertical Reflective Surfaces.
6. Birds Are Often Panicked When They Fly Into Glass.

Source