Why Birds Hit Windows—and How You Can Help Prevent It

For birds, glass windows are worse than invisible. By reflecting foliage or sky, they look like inviting places to fly into. And because the sheer number of windows is so great, their toll on birds is huge. Up to about 1 billion birds die from window strikes in the U.S. each year, according to a 2014 study.

The good news is that you can greatly reduce the danger your home’s windows pose to birds with some simple remedies, according to Christine Sheppard, who directs the Bird Collisions Program of the American Bird Conservancy. The group offers extensive information on preventing collisions on its website. The Fatal Light Awareness Program also offers great information on preventing bird collisions.

What happens to birds that hit windows? Sadly, the bird often dies, even when it is only temporarily stunned and manages to fly away. Many times these birds die later from internal bleeding or bruising, especially on the brain. Daniel Klem of Muhlenberg College has researched this issue since the 1970s. He writes, “Glass is an indiscriminate killer that takes the fit as well as the unfit of a species’ population.”

Why Birds Collide With Windows

Birds typically crash into windows because they see reflections of vegetation or sky, or they see through the glass to potted plants or vegetation on the other side. Collisions can happen at any hour and any season, but are most frequent at dawn and dusk during spring or fall migration.

Many birds migrate at night. At dawn, they fly down to rest, shelter, and fuel up for the next night’s journey. At dusk, they take flight again. In both cases, entering or exiting vegetation puts the birds at risk of mistaking reflections in glass as real habitat and flying into the glass.

At night, artificial lights play a role: they attract migrating birds into urban landscapes, especially in low-ceiling or foggy conditions. Once drawn off course they may roost safely nearby, only to become vulnerable to daytime reflections in windows the following day. The Bird Collision Prevention Alliance and the BirdCast project have more on these issues.

There’s one additional reason: birds sometimes see their reflection in a window and attack it. This is known as “reflection aggression.” It happens most frequently in the spring when territoriality is high. Although it can be annoying to the homeowner, it’s seldom a threat to the bird’s survival. Most of the remedies suggested below for window strikes will also help solve the problem of a bird attacking its reflection.

How to Safeguard Your Windows For Birds

Start by identifying dangerous windows, including large picture windows, paired windows at right angles to each other, or windows with feeders outside. Go outside and look at your windows from a bird’s point of view. If you see branches or sky reflected in or visible through the glass, that’s what the birds will see, too. Past recommendations about safe distances for feeders outside windows are no longer thought to be valid, Sheppard says. “If you’ve got windows near a bird feeder, you should make them bird friendly and don’t worry about how far away they are.”

Treatments for Existing Windows

To deter small birds, markings such as decals on windows should be spaced uniformly 2 inches apart across the entire outer surface of the glass. (This will safeguard the windows for even the smallest birds such as hummingbirds, gnatcatchers, siskins, kinglets, and the like.) Acopian BirdSavers, made of paracord, can be spaced 4 inches apart because of their greater visibility. All marking techniques should be applied to the outside of the window.

• Tempera paint or soap. Mark the outside of the window with soap or tempera paint, which is inexpensive and long lasting. You can use either a grid pattern of 2 inches by 2 inches (see above), or get creative and paint patterns or artwork on your window.
• Decals. Put decals, stickers, sun catchers, mylar strips, masking tape, or other objects (even sticky notes) on the outside surface of the window. These are only effective when spaced very closely (see above). Note that hawk silhouettes do little to deter 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.
• Dot Patterns and Tape. Long-lasting tape products offer an easier way to apply the correct spacing of dots across your window. Products such as those available at Feather Friendly work well in preventing collisions.
• Acopian BirdSavers. Also known as “zen curtains,” these closely spaced cords hang down over windows. They do the work of tape or decals but are easier to install and can be aesthetically pleasing. They are highly effective and are the method we use to safeguard windows at the Cornell Lab headquarters. You can order them to fit your windows or make your own.
• Screens. Installing mosquito screens over your windows is very effective, as long as they are on the outside of the window and cover the entire surface.
• One-way transparent film. Products such as Collidescape permit people on the inside to see out, but makes the window appear opaque on the outside. They can reduce the amount of light that comes in your window (this can also reduce your cooling costs), according to Sheppard.

New Homes and Remodels

• 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. Remote controlled shades are available.
• On new construction or when putting in new windows, consider windows that have the screen on the entire outside of the glass.
• Add interior vertical blinds and keep the slats only half open.
• 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.

Lights Out

Lights Out initiatives are gaining ground in U.S. cities including Dallas, Houston, Philadelphia, and New York. The all-night glow of office buildings and streetlights in cities is especially dangerous for drawing migrating birds off course, delaying their migrations and making them vulnerable to window collisions. Turning off nonessential lights and installing downward-facing lighting are relatively simple ways to reduce the problem of artificial light at night. It’s still wise to take precautions against window collisions using any of the above methods, especially for homeowners.