Birds and Window Glazing

I know that spring has arrived when the little finch outside my home workspace window chirps at its reflection in the window and flits against the glass for hours on end. It is bird mating season, and the little finch is particularly excited by its own appearance and voice. Luckily, the tree branch that the bird rests upon is very close to the window, so when it attempts to fly towards its own reflection it doesn’t injure itself when being stopped by the glass. Instead the bird rests temporarily on the sill and then returns to its tree branch to begin chirping again. The window is north-facing, so there is no direct glare from the sun during the day, but it still has a reflective quality from the outside in the daytime, so the little bird is continuously deceived.

Luckily, I have not experienced any bird injury sightings at this property in the Catskills (other than those being swept up by the red tail hawk for prey). One of the houses that I was previously renting in Tucson, Arizona had very large window walls on the south and north facades of the main living space. The house was surrounded by natural desert habitat on the backyard (to the north) and a very large mesquite tree and oleander bushes in the front yard (to the south). Desert doves would frequently attempt to fly through these closed window walls due to the reflective quality creating a continuous sky and landscape effect. Because they were moving at higher speeds, there were sometimes injured birds resting and recovering outside the windowsills. Eventually I installed some exterior rattan blinds on the north window wall, and some interior curtains on the south window wall, both of which seemed to help reduce the erroneous flight paths for the birds.

The problem with window glass on many buildings is that it is generally deceptive to birds due to reflections of the sky and other surrounding natural features. Thousands of bird deaths occur every day in cities with taller buildings that have primarily glass facades. According to the American Bird Conservatory, between 100 million to 1 billion bird deaths occur each year due to glass collisions, and an equal amount is estimated in other countries such as Germany. Approximately one third of all bird species are documented victims of glass strikes, so it is a wide range of birds that incur these injuries and deaths. There is limited ability for the birds to comprehend nuances of the glass material in comparison with the sky reflections. Because there is no way for the birds to differentiate between sky and reflective glass, building glazing is effectively hazardous for the bird ecosystem. The challenge for glazing technologies lies within the optical sciences to develop appropriate solutions for building glass that can prevent these reflections.  

Building glazing technologies have evolved greatly over the past few decades with advancements specifically developed for building energy conservation and daylighting quality demands. The science and technical capabilities are available but have only been applied in limited examples for solving the reflective issues experienced by birds. Building glass is generally the most ubiquitous exterior wall material around the world and also the most conductive part of the exterior enclosure system. Because glazing is the weak point for building heat transfer processes, the heating and cooling loads for a building are more greatly affected by the windows compared to other parts of the exterior walls. When more stringent building energy codes and standards emerged, glass technologies responded with advanced spectral selectivity and optical control in order to reduce heat gains or losses.

Lawrence Berkeley National Laboratories (LBNL) has a vast database of glazing systems and glass types that are tested and verified for their optical parameters. There are over 1,000 glass types in that International Glazing Database (IGDB), which provides designers with the ability to simulate both the daylight and heat transfer properties with building energy analysis tools. It seems inherently obvious, from the standpoint of ecological ethics, that glass technology should also be developed to appropriately deal with bird vision and interpretation to alleviate the problematic and deadly interferences of flight paths. Indeed, there are a couple of bird protectant glazing technologies now on the market, one that is produced from Pilkington (US and Canadian company) and another by ArnoldGlas (a Dutch company), but this is a very small offering amidst the thousands of glazing types available.

The current bird protectant glass that is available on the market focuses on the use of UV-reflecting patterns deposited onto the outermost surface of the window glass. Ornithologists describe the vision functions of birds as having four cone cells, rather than three (RGB) that humans have. In addition, one of the maximum absorption peaks (for the electromagnetic spectrum) extends to the UV wavelengths between 300-400nm for birds. Humans can only perceive light information within the visible spectrum of 380nm–740nm wavelengths. Because of this, bird protectant glazing technologies make use of UV-reflective coatings, which birds will notice but which will remain largely invisible to the human eye. This advancement handles the general challenge of having visible patterns showing too greatly on the windows, which otherwise is perceived as distracting for clarity of human views.

The truth is that there are millions of windows already installed in the built environment, and it would not be viable to replace all the existing building glass with bird protectant types. So, there are some existing off-the-shelf solutions that can be used to treat existing windows. Window decals are the common go-to option for homeowners and renters of residential properties. Decals are the most affordable and can also be removed without leaving damage. Some things to consider for making decals effective for preventing birds from striking your windows is that the decals must be applied to the outside surfaces of glass, especially if these are UV-reflective (most recommended for effectiveness). Another thing to consider is the spacing and placement of the decals upon the window. Based on research studies of bird flight and the frequency of birds flying through different passages, an apparent opening of 2-inches height or less and 4-inches wide or less will deter the bird’s flight. If you space the decals too far apart from each other, the birds may not acknowledge these patterns as deterrents to their flight path. In addition, decals can perhaps appear a little tacky and decorative in a distracting way for your own aesthetic preferences.

Adding exterior blinds or shutters with operable slats to your windows can be one of the first best measures to prevent birds from flying directly into your home’s glass surfaces. Of course, you still will want to allow daylight into your home through the windows, so the best type of blinds will be ones that you can pull down slightly from the top and pull up slightly from the bottom, leaving the mid-range of the window covered. Slatted blinds or shutters can also suffice since these can be tilted to allow in natural daylight while still covering the full surface of glass to deter birds. This will only work to deter birds when the blinds or shutters are placed on the exterior side of your windows – but the second benefit to this is that you will also help to reduce heat gain through your windows.

There are also some bird-protectant window films on the market that utilize the new UV-reflective technology, providing very effective relief for bird injuries and alleviating the obfuscation of views and visibility for the human eye. The one which I found to be a great option for bird protectant window films is made by a company called Decorative Films. They have a few different patterns with the UV-reflective films, each which provide the appropriate spacing and design for preventing bird strikes. These seem to be very non-obtrusive compared to other individual decal options and will ensure effectiveness for bird visibility. The challenge for these types of window films can be with the installation, since they are produced in sheets (cut from rolls of film) that need to be applied to the exterior glass in one pass. It is best to have professionals do the installation to avoid bubbles or misalignment and especially if you have second-story or high windows on the exterior that require a ladder and extensive reach.

Another alternative is to create transition zones between the outside walls of your house and your yard with shrubbery and plantings that grow slightly in front of window surfaces. The natural flora will prevent direct flight paths into your windows, but as demonstrated by the little finch outside my home’s workspace window, the birds may still notice their own reflection in the glass when perched on surrounding plants and trees. A more costly trade-out is to install new glazing in your home’s windows. This could be done with anti-reflective glass or bird protection glazing, though the latter is currently of limited availability through only a couple of manufacturers mentioned earlier. You could also install fritted glass, which creates a white-washed effect with various patterns on the outside that is drastically less reflective. The frit patterns can also benefit your home’s utility bills during hot seasons by reducing heat gain while still providing good quality natural daylight.

If you are planning any window replacements for your house and want to know the best selection for your location and climate, reach out to AIDA, LLC today for a consultation. You can always find more information and healthy home resources at Aletheia Ida Design and Architecture, LLC (AIDA, LLC) at www.aletheiaida.com.