By Mark Hostetler and Elle Henson

Glass windows are one of the most common causes of bird mortality in North America. Especially during the migration season, millions of birds are flying around buildings and unfortunately many hit windows. Estimates suggest that hundreds of millions to over a billion birds die every year after flying into windows.

Why It Happens

Birds rely heavily on visual cues to navigate through a landscape. However, highly reflective windows trick them. A window can mirror sky or vegetation or appear to be open space, leading to high-speed collisions that are often fatal. In particular for migrating birds, they often migrate at night and are drawn to urban areas by lights. Many will collide with tall buildings at night and for those that remain, taking flight in the morning may result in hitting reflective windows.

Common risky situations include:

• Windows near trees, shrubs, feeders, or bird baths — reflections can look like real habitat.
• Large panes of glass with few visual features, where birds see uninterrupted reflections.
• Windows along a building corridor, which are positioned in a way where there is a clear visual of habitat on the other side and birds think they can fly through it.

• Windows in well-lit areas, where birds may be drawn in at night-time by the artificial lights and then during the day, hit reflective windows.

Solutions That Work

Fortunately, there are simple, effective ways to reduce bird strikes and homeowners can implement them on a budget.

1. Break Up Reflections on the Outside of a Window

Birds must recognize that windows are a solid barrier. That means treatments must be on the outside of the window .

Effective ways to do this include:

• Patterns with paint, tape, or markers. One can draw or place stripes, dots, or other shapes with spacing no greater than 2 inches vertically and 4 inches horizontally (for smaller birds, 2”×2” is even better).
• UV-reflective decals or films designed specifically for birds. They show up to birds even when nearly invisible to people.
• External screens or netting, which break up reflection and physically block flight paths.
• Hanging thin ropes from the top to the bottom of the window (see Acopian Birdsavers)

Note: Small decals or a couple of random stickers won’t help the because the window must be covered with a pattern birds can’t “fly between.”  For more details, see American Bird Conservancy – https://abcbirds.org/solutions/preventing-collisions/ .

Acopian BirdSavers: An Easy, Cost-effective Solution

One of the most recommended and bird-friendly solutions is a method called Acopian BirdSavers.

What They Are (https://www.birdsavers.com/)

Acopian BirdSavers are hanging cords placed just outside windows. They consist of vertical lines of weather-resistant cords, traditionally nylon or paracord, spaced roughly 4 inches apart.

Why They Work

• Visible physical cues: Instead of relying on patterns on glass, the cords sit in front of the window. Birds recognize these as obstacles and avoid flying into the glass.
• To people inside a home, the cords are thin and often unobtrusive. Many homeowners find they “fade into the background” after a short time. Further, they are typically attached with Velcro and can be taken down and put up easily.
• When cords are no more than four inches apart, BirdSavers help deter collisions for everything from hummingbirds to larger songbirds.

DIY or Buy

You can easily make your own BirdSavers using basic paracord and mounting brackets from a local hardware store, or you can purchase them pre-made for a modest price. (https://www.birdsavers.com/)

Overall Best Practices to Reduce Window Collisions

Position Bird Feeders Carefully

• Place feeders very close (within ~3 feet) to windows OR far away (>30 feet) to reduce high-speed approaches. Birds flying away from feeders close to glass don’t build up as much speed. More than 30ft, birds have space to recognize the window as a hazard.

Treat All Hazardous Glass

• Apply patterns, films, or BirdSavers to every pane that might be confusing or reflective, partial treatment leaves gaps birds may crash through.

Reduce Artificial Lighting at Night

• Especially during migration seasons, lights attract and disorient birds. Turning off unnecessary lights or using motion sensors can help.

Keep Vegetation in Mind

• Trim plants and avoid placing shrubbery right up against windows because reflections of greenery can look like real cover.

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