The following information is from the Cornell All About Birds website.
Window strikes are something you should be aware of and try to prevent; you have an added responsibility if you feed wild birds in your backyard. Start by identifying dangerous windows. Large picture windows or a pair of windows at right angles to each other on the corner of a house or other building 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 or through the glass, that’s what the birds will see, too.
Try some of these ideas to make your windows safer:
- Cover the glass on the outside with window screening or netting at least 2-3 inches from the glass, taut enough to bounce birds off before they can hit the glass. This can be extremely effective. The Cornell Lab of Ornithology installed crop netting—the kind used to keep birds away from fruit trees—in front of a large picture window next to our bird-feeding garden. The result? No more dead and injured birds. Small-mesh netting is best—ours is 5/8″ (1.6 cm) in diameter—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.
- Cover the glass with a one-way transparent film that permits people on the inside to see out, but makes the window appear opaque on the outside. You can find information about the best available products on the Fatal Light Awareness Program website. Make sure these kinds of products are mounted on the OUTSIDE of the glass.
- Place a wooden grille or vertical tape strips on the outside of the glass, set not more than 10 cm apart, or mark the glass with soap or permanent paint in the same way. (The inks in most markers usually degrade in sunlight very quickly.)
- Install external shutters and keep them closed when you’re not in the room or taking advantage of the light or view. (These can be huge energy savers, too!)
- Install external sun shades or awnings on windows, to block the reflection of sunlight.
- On new construction or when putting in new windows, consider double-hung windows, which have the screen on the outside of the glass. Alternatively, you might be able to ask your contractor to construct the window so the glass angles downward and doesn’t reflect sky and trees. Unfortunately, in some cases this may void the warranty on the window.
- Put decals, stickers, sun catchers, mylar strips, or other objects on the outside surface of the window. These are only effective when spaced very closely—no more than the span of a large hand between them. The design of a decal or sticker is immaterial. Hawk silhouette stickers are probably no more or less useful than any others. Some stickers sold in bird-feeding stores are colored in the ultraviolet spectrum—these appear transparent to our eyes but are visible to birds. Remember: placing just one or two window stickers on a large window is not going to prevent collisions—they must cover most of the glass with the spaces between too narrow for birds to fly through.
- Keep the slats only half open on interior vertical blinds
- Relocate feeders and other attractants. Without changing your window, you might be able to reduce mortality, at least by resident birds, by moving your feeders and birdbaths to new locations. Bird strikes are significantly more likely to be fatal when they take off far enough away from the window to be flying at top speed when they hit. When feeders are placed within 3 feet of window glass, or affixed to the window or frame, birds may still fly into it, but seldom with enough force to injure themselves.
- Avoid visual paths to sky and greenery. Bright windows on the opposite wall from your picture window may give the illusion of an open path to the other side. Closing a window shade or a door between rooms can sometimes solve this situation.