Did you know?
Collisions with building windows are a major direct source of bird deaths.
Around 25 million birds in Canada and up to 1 billion birds across North America are killed by colliding with windows each year.
There are currently no estimates of how many birds are killed in London each year, but based on available data it is likely in the range of hundreds of thousands.
At least 80 bird species are affected by collisions with windows in London.
In Canada, around 90% of bird-window collisions happen at residential homes. Only a small fraction of collisions take place at tall buildings (source)
Most bird-window collisions happen during the daytime, but some collisions happen at night, especially where bright lights are left on during bird migration.
Bird-window collisions can be prevented using simple, inexpensive solutions for existing buildings and new construction.
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Why do birds crash into windows?
Birds usually crash into windows on buildings because they mistake a reflection on the glass for an extension of their habitat or open space. Birds will try to fly through the window to reach the reflection. Reflections on vertical surfaces do not exist in nature and birds do not understand them!
In some situations, glass appears transparent and birds may not see it. For example, glass railings on balconies or walls on transportation shelters are invisible to birds.
Some bird-window collisions happen at night, especially near tall buildings. The risk of nighttime collisions seems to be related to the amount of light pollution nearby.
The risk of birds crashing into glass varies between windows, between buildings and across time. Collision risk is highest where windows are located near green space or areas where there are bird present, such as backyards with bird feeders.
Birds pecking or flying at windows repeatedly
Birds do not understand the concept of reflections. When a bird sees its own reflection on a window, they mistake it for another bird. During spring, many bird species stake out and defend breeding territory. If a bird spots another bird in their territory, they may respond by attacking it. This is typically how birds end up in fights with their reflections on building windows. Unfortunately, this behaviour can drag on and birds can expend a lot of precious energy and make themselves vulnerable to predation while attending to their rival in the mirror.
To stop birds from attacking windows, you will need to mute or remove the appearance of reflection on the glass. With their reflection removed, most birds will stop surveying and attacking the window within a couple of days.
If the window can be accessed from outside, try adding something to the exterior surface to blur or obstruct the reflection. You may use soap, chalk, cornstarch, washable paint, newspaper or anything else readily available. Note that some materials might need to be reapplied.
If the window can only be accessed from inside (e.g., on upper stories) different solutions may be applied. One option is to put something light in colour behind the glass, like a curtain. Try taping paper or a sheet over the inside of the window. An alternative is to shine a bright light (a lamp or flashlight) towards the glass from inside. Any of these methods will mute the reflection on the outside.
You might see a bird peck, fly at or attack the same windows repeatedly. This behaviour is especially common in spring and can drag on for weeks, becoming quite stressful for both birds and building occupants. The causes of this behaviour and solutions are different from bird-window collisions discussed above. In most cases birds will stop this behaviour on their own and will not get hurt.
Note that this bird is not flying beak-first as if trying to pass through; it is attacking what it sees as another bird.
Preventing bird-window collisions
There are a variety of options available for making residential and commercial building windows safer for birds. However, there are also products that, despite marketing claims and popularity, are not effective for preventing bird collisions (see below).
Bird Friendly Windows
Most bird-window collisions occur during daytime, either because the bird mistakes the reflection on the glass for an extension of habitat or open space, or because the bird does not see glass and tries to fly through it to reach the other side.
The best methods to prevent collisions involve modifying the appearance of a window so that birds will be able recognize it as a solid object they can't fly through and avoid crashing into it.
A variety of materials can be used to reduce the risk of bird-window collisions at new and existing buildings. We recommend sticking to solutions that have been endorsed by authorities based on scientific evidence for their effectiveness. Window treatments for preventing bird-window collisions should comply with the Canadian Standards Association (CSA) 2019 Bird Friendly Building Design standard.
Gaps between window markings should not exceed 5 cm by 5 cm (2 inches by 2 inches). If gaps are any larger, birds may try to fly through them and still hit the window.
Apply markings to the outside surface of the glass, NOT the inside. Reflections of trees or sky on the outside of the window may render any internal window markings invisible.
Markings must provide high contrast so that they stand out on the window. Markings with poor contrast, for example black dots on a very dark window, might not be noticed by birds.
Each marking size should be no less than 6 mm (1/4 inch) in diameter.
Markings must cover the entire surface of the glass.
Source: FLAP Canada (2022)
The organizations linked below are considered leading authorities on bird-window collisions in North America. They provide recommendations for bird-friendly window materials both for retrofitting existing windows and for new construction:
American Bird Conservancy: Bird Smart Glass. A database of collision products that have been tested for effectiveness.
What does NOT work (according to experts and available scientific data)
XSingle visual markers like bird silhouettes or individual decals will not work if you only apply a few on the glass. There is little evidence that birds will avoid windows with stickers or objects shaped like predators.
The reason is because birds will simply fly around the markers and hit uncovered parts of the window. Many bird species are quite good at flying through crowded areas and squeezing between obstacles.
See below for information about proper marker spacing (note this will require many markers)
A northern goshawk effortlessly closes its wings to fit between tree trunks. Imagine birds doing this as they try to fly through gaps between visual markers on a window.
XCovering the inside of the window does not break up the appearance of reflection on the outside of the glass. Similarly, closing blinds or curtains might help in some cases where windows appear transparent from the outside, but they will not break up the appearance of reflections on windows that cause most bird-window collisions. If possible, any window treatments such as installing visual markers should be applied on the window exterior.
XUltraviolet (UV)-based products for preventing bird-window collisions, such as Window Alert decals, stickers and ink, are NOT recommended. There is little scientific evidence to support that these products are effective for preventing bird collisions. Although many bird species can see some amount of light in the ultraviolet spectrum, it is unclear how birds actually see UV treatments on windows. The performance of UV products under variable outdoor lighting conditions is poorly understood. According to the manufacturers, UV decals and ink need to be replaced at least every 4 months for the UV signal to be maintained, making this option quite expensive and tedious. If markers are spaced appropriately on the window, an installation would require too many stickers for these products to be economical.
XBird deterrents like decoy statues provide mixed effectiveness for stopping birds from approaching building windows. Most birds can quickly learn to distinguish fake from real predators. If a bird is flying towards a reflection in a window, tricking the bird into thinking a predator is nearby will not help them to recognize the window does not provide a flight path to escape.
A note about deterrent effectiveness: To prevent bird-window collisions and save birds, it is important to use techniques and products whose effectiveness is supported by available scientific evidence and consensus among experts. If we treat windows but fail to help birds to see and avoid crashing into them, what is the point?
Maybe you have tried one of the "ineffective" methods described above and found some success with reducing bird-window collisions. It is important to remember that people, including homeowners, are not able to reliably measure the frequency of bird-window collisions. In fact, many bird-window collisions do not cause a detectable sound, leave behind a mark on the window or result in the bird being immediately grounded or killed. The number of bird collisions that occur can vary over time due to a variety of factors besides the treatment applied to the windows, but these cannot be accounted for through personal observation. Thus, it is best to follow advice from experts and adopt only trusted, demonstrated solutions for preventing bird-window collisions.
Information for Homeowners or Renters
According to peer-reviewed research by Machtans et al. (2013), in Canada over 90 percent of bird-window collisions happen at single-family homes.
Birds are most likely to collide with windows near green space (habitat), and usually collide with windows at or below the height of surrounding trees (i.e., up to the 4th floor).
Glass railings are another major source of bird collisions at homes, because birds fail to detect glass that is invisible.
A variety of window retrofit solutions are available for homeowners or renters in London to reduce the risk of bird-window collisions.
Suggestions for DIY Home Window Retrofits
Feather Friendly DIY Tape (create a grid of white or black dots) is available to purchase locally in London at:
Featherfields Bird and Garden Store (Hyde Park, Wortley Village)
Heeman's Garden Centre (East London)
Birders Bird Supply & Gift Store (West London)
Oil-based markers, paint, tape or stickers can be used to add visual markers to the exterior of a window. Here are suggestions for materials to use (read more from our blog)
Information for Businesses
For larger projects such as large residential or commercial window retrofits, or including bird-friendly glass in new construction projects, local London-based companies that offer bird-friendly solutions include:
Window Film Systems (official Feather Friendly supplier for London area)
The London Bird Team organizes volunteer-driven window retrofit events. If your business is interested in hosting an event and having your building windows treated by volunteers, please contact us! We prioritize window retrofits in public-facing spaces, especially near natural areas.