At Audubon California, we are often asked about preventing bird collisions. It is estimated that around 3.5 million birds die each year from strikes in the U.S. alone. Here are some tips to reduce this number:

• Keep bird feeders and water features away from windows:  Feeders and water features should be at least 0.3 meters away from windows to allow birds to slow their momentum enough to avoid contact;
• Cover windows with netting:  Naturally occurring spider webs also work as a deterrent;
• Design new windows at an angle: These will generates a reflection on the ground that the bird can see;
• Apply decals to windows: Watch above video from the Wisconsin Humane Society for details.

Source

Klem, Daniel Jr. "Collisions between birds and windows: mortality and prevention," Journal of Field Ornithology: 1990,  61(l):120-128.