Ornithologists estimate that up to 100 million birds are killed each year by collisions with windows. These collisions usually involve small songbirds, such as finches, that may fall unnoticed to the ground. Sometimes the birds are merely stunned and recover in a few moments. Often, though, window hits lead to severe internal injuries and death.
It's thought that birds hit windows because they see the landscape—trees, sky, clouds—reflected on the glass surface but do not realize that a hard, transparent surface lies between them and that apparent open space. Panicking birds, fleeing for cover to escape predators, are even more likely to fly into windows.
If you find a bird dazed from a window hit, place it in a dark container with a lid such as a shoebox, and leave it somewhere warm and quiet, out of reach of pets and other predators. If the weather is extremely cold, you may need to take it inside. Do not try to give it food and water, and resist handling it as much as possible. The darkness will calm the bird while it revives, which should occur within a few minutes, unless it is seriously injured. Release it outside as soon as it appears awake and alert. If the bird doesn't recover in a couple of hours, you should take it to a veterinarian or wildlife rehabilitator. Remember that, technically, it is illegal to handle a migratory bird without a permit.
Window strikes are something you should be aware of and try to prevent, especially if you feed wild birds in your backyard. Start by identifying which window is the problem—large picture windows are usually the worst culprits. Go outside near your feeders and look at your windows from a bird's point of view. If you see branches or sky reflected in the glass when you look at your window, the birds can as well. Can you see through the window into the house? If so, the birds can too. Is there another window on the opposite wall of the house? It may give birds the illusion of a fly-through passage to the habitat outside.
To make windows safer you can start by simply moving your feeders and birdbaths to new locations. Bird strikes usually occur at particular windows, so moving feeders farther away from them may solve the problem entirely. You can also try placing your feeders much closer to the glass. If a feeder is just a foot or two from a window, birds may still fly into it, but not with enough force to injure themselves.
Bright windows on the opposite wall from your picture window may give the illusion of a visual tunnel through which birds may try to fly. Try making one window less transparent by keeping a shade drawn or a door closed, or by altering the lighting inside the house. You can also make the glass less transparent by taping paper or cardboard on the inside of the panes.
Break up window reflections by sticking objects to the outside of the glass. Black plastic silhouettes of a falcon, hawk, or owl sometimes work, not because they look like predators but because they disrupt the window's reflectivity. Semi-transparent stickers can also do the job some have decorative bird shapes, or look like spider webs. Sheets of plastic food wrap may work too.
Try spraying fake Christmas snow on the outside of the window, or drawing streaks across it with bar soap. Again, the goal is to break up external reflections.
For a more natural look, attach dead tree branches in front of your window. They may cause the birds to slow down and avoid the window as they fly toward it. You can arrange the branches so they don't obscure your view.
Hang lightweight, shiny items in front of the window so they move in the breeze and dissuade birds from approaching. Try strips of shiny, reflective plastic (hung a few inches apart), old aluminum pie plates, or unwanted compact discs.
Reduce the amount of light reaching a problem window by planting shade trees close to it. This will help prevent reflections. However, it will also obstruct your view. Trees take time to grow, so consider shading your window with an awning instead. Either one may help birds by reducing the amount of sky reflected in windows.
Place netting over the window. It provides a physical barrier to birds flying into the glass, yet won't obstruct your view. The Cornell Lab of Ornithology installed crop netting of the kind used to keep birds away from fruit trees in front of a large picture window next to our bird-feeding garden in our original building. The result? No more dead and injured birds. Small-mesh netting is best so if birds do fly into it they won't get their heads or bodies entangled but will bounce off unharmed. You can mount the netting on a frame, such as a storm-window frame, for easy installation and removal. You could also try insect screening material.
If you're installing new windows, ask your contractor to position them slightly off vertical, facing downwards. Then the outer window surface will reflect the ground rather than the sky and trees, but won't affect your view from inside the house. Be aware, though, that this may void your warranty. Your contractor or architect may have other useful ideas about how to minimize habitat reflection in your windows.
Reprinted from All About Birds, a publication of the Cornell Lab of Ornithology.