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

April 20, 2009
american goldfinch killed by window strike Photo by Susan Spear/Cornell Lab.

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 recent study.

Often a collision will temporarily stun a bird but it will fly off, seemingly recovered, a few moments later. Many times these birds die later, from internal bleeding or bruising, especially on the brain. Dr. Daniel Klem of Muhlenberg College has researched this issue since the 1970s. He writes, “Intensive studies at single homes reveal one out of every two strikes results in a fatality.” Klem adds, “Glass is an indiscriminate killer that takes the fit as well as the unfit of a species’ population.”

imprint of dove on plate glass windowThe window imprint left by a Mourning Dove. Photo by Priscilla Bradley/PFW.

Why Birds Collide With Windows

There are two main types of window collision mortality. The first happens by day, when birds crash into windows because they see reflections of the landscape or see through the glass to potted plants or vegetation on the other side. The second type happens by night, when nocturnal migrants (including tanagers, orioles, thrushes, catbirds, and warblers) hit lighted windows that jut into their airspace. Some of these nighttime collisions are due to simple chance, but much more often the nocturnal migrants are lured to their deaths by the lights. For reasons not entirely understood, lights divert nocturnal migrants from their original path, especially in low-ceiling or foggy conditions. In the lighted area, they mill about, often colliding with one another or the lighted structure. The Fatal Light Awareness Program, based in Toronto, Canada, has much more about this problem.

reflection of foliage in window - hazard to birdsReflected landscapes can confuse birds and cause deadly window strikes. Birds may also attack their own reflection, believing it to be a rival. Photo by Susan Spear/Cornell Lab.

Why Birds Attack Windows

A third type of window collision is far less likely to end in mortality, although it can be annoying to humans—that’s when a bird attacks its reflection in a window. This occurs during daytime, most often in spring and early summer when birds are defending their breeding territories. Some of the species that do this most often are Northern Cardinals, American Robins, and California Towhees. It happens when a bird notices its reflection and perceives it as a territorial rival. Territorial battles with windows may be so strong that a bird may exhaust itself, but these collisions usually don’t result in fatal injury.

The best solution, which will also prevent fatalities from other kinds of window collisions, is to cover the outside of the window with netting or screening so the reflection is no longer visible or the bird is held too far from it to hurt itself or be such a nuisance for you. You can also try drawing soap streaks across the window to break up the reflection.

guidelines for how close to put bird feeders to windows to avoid deathsPlacing feeders close to windows helps to avoid deadly collisions. Image by Cornell Lab.

Safeguarding Your Windows For Birds

Window strikes are something you should be aware of and try to prevent. Start by identifying dangerous windows. Large picture windows or a pair of windows at right angles to each other 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:

hawk or falcon decals are mostly ineffective at deterring window strikesUnfortunately, window decals rarely prevent bird strikes.
  • 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 more likely to be fatal when birds take off far enough away from the window to be flying at top speed when they hit. Feeders within 3 feet of window glass, or more than 30 feet away are the safest.
  • Cover the glass on the outside with window screening or netting at least 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 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. 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 4 inches apart, or mark the glass with soap or permanent paint in the same way.
  • 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 slightly 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
  • 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.

How to Help a Window Collision Victim

If you find a bird dazed from a window collision, examine it for external injuries. If the wings are both held properly, neither dangling, and the eyes seem normal, see if it can perch in a branch unassisted. If so, leave it to recover on its own.

If the bird has a noticeable injury, get it to a wildlife rehabilitator as quickly as possible. Broken bones usually need proper attention within minutes or hours to heal properly without surgery. Use this online directory to find a rehabber near you.

Meanwhile, place it in a dark container such as a shoebox, and leave it somewhere quiet, out of reach of pets and other predators, for 15 minutes. If the weather is extremely cold, you may need to take it inside, but don’t keep the bird too warm. Do not try to give it food and water, and resist handling it. The darkness will calm the bird while it revives, which should occur within a few minutes unless it is seriously injured. Do not open the box indoors to check on it or it might escape into your house and be hard to get back out!

Take the box outside every 15 minutes or so and open it—if the bird flies off, that’s that! If it doesn’t recover in a couple of hours, take it to a wildlife rehabilitator. Remember that, technically, it is illegal to handle a migratory bird without a permit, and medically helping an injured bird requires training, so your job is just to transport the bird to a rehabilitator.


  • Elizabeth Barrera

    Try CollidEscape. I just did. 100% guaranteed to prevent another bird from striking windows.

    Call Jeff Rank directly and save these birds’ lives. 830-255-7265