One of the leading causes of death for wild birds is flying into glass windows. A study conducted by Daniel Klem Jr. at Muhlenberg College, estimated that 97 million birds die each year in the U.S. as the result of collisions with windows. Tall buildings that relied heavily on large sheets of glass surface were especially hazardous.
Birds see a reflection of the outdoors in the window and do not realize they are flying into a solid object. The end result is a stunned bird or a bird with a broken neck.
Birds see their own reflection in the window
During the spring, birds will appear to attack windows. They are really defending their territory from the "other" bird they see reflected in the window. They lunge at the "other bird" and peck it, trying to drive it away. So that "crazy" Cardinal is really not so crazy -- he is just defending his territory (and his territory includes your house!)
To solve the window-kill problem, we need to tell the birds that there is a solid object in their path. This can be done from the inside or from the outside. We can also reduce the problem by moving the birds farther away from the window.
Your windows are acting like mirrors when your house is dark. Turn on lights inside your house. Otherwise, they will only see their own reflection or the reflection of the trees behind them. You want the birds to be able to see into your house. Altering drapes or blinds may also help reduce the reflection. Adding curtains to a window on the other side of the room might remove the "tunnel" birds see when looking straight through your house.
You can reduce collisions by placing screen, netting or strips of cloth outside your windows as a warning. Hawk decals will work, but only if you use a lot of them as a way to turn an invisible window into a "solid wall". Actually any decal or paper cutout would work. One or two big hawk decals are useless.
If you live in an area with lots of migrating birds, you may want to put up a barrier in front of your windows for a week or two while they are passing through.
Go outside and look at your windows. Check in the early morning, at noon and at dusk. Imagine you are a bird. Can you see your own reflection or the reflection of the trees behind you? Would you know there was a window in your way?
If a hawk swoops down on your bird feeder, all the birds will scatter as fast as possible. Some may fly directly into your window! By moving the feeder farther away from the window, the birds stand a better chance of surviving. Why not plant a thick evergreen near the feeder so the birds have someplace to hide.
Skyscraper windows in the big cities are a very big problem for migrating birds. This is because the light from skyscrapers, lighthouses and other tall structures attracts migrating birds. The birds may flutter around the light until they drop from exhaustion or they collide with the structure. In large cities, they may find themselves trapped in a maze of bright office towers, unable to escape. A group called Fatal Light Awareness Program is addressing these issues.