When you dream of living close to nature, you probably picture a beautiful home with big windows looking out over the forest, and birds all around you. What most of us don’t think about when we design or buy a home is what all that glass means for those birds. They can’t see glass the way we do, and many of them fly into windows.
The good news is that it’s completely preventable. At Ecovilla San Mateo, our intentional community in Costa Rica, we live surrounded by forest and birds, so we asked the experts at American Bird Conservancy what really works. Here’s what we learned.
In short: birds fly into windows because they see a reflection of trees and sky, or see straight through the glass to the green on the other side. You can prevent it by making the glass visible to them, with dots, cords or screens on or in front of the outside of the window. In Costa Rica, keep the gaps to about 5 cm (2 inches), so even hummingbirds can’t fly through.
Why birds fly into glass
Bird populations around the world are declining, and collisions with glass are one of the biggest human-caused threats they face. It’s often seen as a city problem, but most collisions actually happen at small buildings like our homes. Any species can be affected, all year round.
The cause comes down to two things:
- Reflection. Any glass, even so-called non-reflective glass, can turn into a perfect mirror of trees and sky in the right light. Birds take what they see literally.
- Transparency. When birds see through glass to plants, cover or sky on the other side, they try to fly there.
We need very little to notice glass, a few small dots on a door are enough. Birds see the world differently and need a much more obvious pattern. And when a bird flies off after a hit, that doesn’t mean it’s okay. Many victims also leave no trace, so we only ever see a fraction of the problem.
Which homes are most at risk?
- More glass means more collisions. Large picture windows with forest views are beautiful, and they’re also the biggest risk.
- Homes surrounded by vegetation. We want all that green around us, so the solution is to treat the glass.
- Corners, indentations and parts that stick out. Collisions tend to concentrate in certain spots of a building.
- Flyways. Birds use open paths, like the path leading up to a glass front door.
- See-through spots. Windows on opposite sides of a room, or glass railings and panels that stand on their own.
What doesn’t work
- A decal or two. A single bird sticker only tells birds not to hit that exact spot. They fly right past it into the rest of the glass.
- Overhangs and angled glass. Angling glass is expensive and doesn’t solve the problem.
- Dark tinted glass. It can make reflections worse.
- UV dots. They’re invisible to us, but doves, pigeons and birds of prey can’t see ultraviolet either, and the dots only work in bright light.
- Non-reflective glass on its own. It still reflects around 7 to 8% from the outside, so in some light it can still act as a mirror.
Three ways to make glass visible to birds
- Markers on the glass. Dots, film, etching or patterns on the glass itself, ideally on the outside surface.
- Markers in front of the glass. Cords or strings hanging in front of the window.
- A physical barrier. Screens, shades or a trellis with climbing plants that keep birds from reaching the glass.
Whatever you choose, size and spacing matter most. Research by American Bird Conservancy shows how big a pattern needs to be, and how close together, before birds see it as a barrier:
| What | Size and spacing |
|---|---|
| Dots or etched patterns | Dots of about 6 mm (1/4 inch), on the outside surface of the glass |
| Standard spacing | 5 cm (2 inches) apart in one direction and 10 cm (4 inches) in the other |
| Spacing in Costa Rica | 5 by 5 cm (2 by 2 inches), for dots, cords and fishing line alike |
Why closer in Costa Rica? Most hummingbird species have a wingspan of 10 cm (4 inches) or less, and they know their body size perfectly. With 10 cm (4 inch) gaps, they can still fly through and hit the glass.
Proven solutions for your home
Dot films (such as Feather Friendly)
Small dots applied to the outside of the glass. Only the dots stay on the window, and after a short while you hardly notice them. Choose the color for your situation: black dots where birds can see through the glass (for example a room with windows on both sides), white dots where the glass mainly reflects trees and sky. Any color is better than nothing, and even a less-than-ideal color cuts collisions by at least half. A local window-tint installer can often apply the film, and on a new build it can sometimes be applied before the panes are installed.
Paracord curtains (Zen wind curtains)
Vertical cords hung in front of the window, about 5 cm (2 inches) apart. Gaps of 12 cm (5 inches), which you sometimes see at eco-lodges, are too wide. They score very well in testing, move gently in the wind, cover large windows and are easy to make yourself. One family in our community hung them after losing many birds in their first years, and they haven’t lost a single bird since. They say they don’t even notice the cords anymore.
Fishing line (monofilament)
Cheaper and less visible than paracord. It was tested at 7.5 cm (3 inch) spacing and scored lower than paracord, but it probably works better in real sunlight, when the line catches the light. To make it, fix a strip with holes drilled every 5 cm (2 inches) above and below the window and string the line vertically between them.
Screens and shades
Outside window screens are one of the best solutions wherever they fit your home. One household in our community noticed that their windows with screens or decorative security bars never caused a collision.
Bird-safe glass for new builds
- Laser-etched glass: a permanent pattern of dots of about 6 mm (1/4 inch) etched on the outside surface, at 5 by 5 cm or 5 by 10 cm (2 by 2 or 2 by 4 inches). Vitro’s Bird Smart glass is tested and recommended by American Bird Conservancy, and Cardinal makes a similar laser-etched glass that it sells to window manufacturers such as Andersen and Pella. In Costa Rica it may need to be imported from the United States, which adds cost.
- Fritted glass: a ceramic pattern baked into the glass. It’s permanent and also gives shade, though the pattern usually isn’t on the outside surface. Light gray frit works like white dots, dark gray like black.
- Perforated film: printed film that looks see-through from the inside and opaque from the outside. It also adds privacy and shade, and is mostly used on larger buildings.
Bird-friendly solar film
If you were planning to tint your windows anyway, look for a solar film with a textured dot pattern, such as the bronze-tinted bird-friendly films from Saint-Gobain Solar Gard and Decorative Films. You get sun protection and bird protection in one.
Quick, temporary fixes
Washable tempera paint, applied with small dauber brushes in a dense pattern of dots or lines no more than 5 cm (2 inches) apart, helps right away and buys time for a permanent solution. For a single small window, a creative pattern or even origami birds can work. At Ecovilla San Mateo, we’re applying bird-safe patterns to the windows of our office buildings as well.
Building or designing a home?
- Still in design: this is the best moment. Avoid the riskiest features and choose a proven bird-safe solution from the start. Ask your architect to specify bird-safe glass from the start, the same way you’d choose the kitchen you actually want.
- Glass already ordered: dot film can sometimes be applied before the panes are installed, which saves you scaffolding later.
- Existing windows: retrofits like film, cords and screens are usually the most affordable option.
Architects can follow a one-hour online course by American Bird Conservancy, so share it with yours.
One more tip: a long butterfly net
With open homes, birds, bats and big butterflies sometimes fly inside and can’t find their way out, especially under high ceilings. A large butterfly net on a telescoping handle lets you guide them out gently without hurting them.
Learn more
American Bird Conservancy has tested hundreds of glass products in its testing tunnel and shares everything homeowners need to know: Preventing Glass Collisions, bird-safe solutions for homes and a database of bird-friendly products.
Would you like to discover more about living surrounded by nature at Ecovilla San Mateo? Get in touch with us and come and see it for yourself.