Birds flying into windows is a common phenomenon that many homeowners experience. While it may seem strange to us, there are several reasons why birds collide with windows repeatedly.
They See Reflections
One of the main reasons birds fly into windows is because they see reflections in the glass. To us, a window is clearly a solid barrier. But to birds, the reflections of trees, skies, and other features look like a clear flight path. So they try to fly right through what looks to them like an open space, only to hit the glass.
This reflection effect is compounded when windows are larger, cleaner, more mirror-like, and have habitats or food sources visible on the other side. All of these factors make birds more likely to keep trying to fly into the reflections, thinking they will make it through.
They Don’t See the Glass
In addition to reflections, birds also crash into windows simply because they don’t see the glass. Windows, especially newer models, can look perfectly transparent from a bird’s perspective during the day. The sheer invisibility of glass is a major cause of window collisions.
Birds’ eyes have better color vision and ability to see ultraviolet light than human eyes. But their distance vision is not as acute. So a bird can be flying straight towards a window and not realize the glass surface is solid until it is too late.
Disorientation from Transparent Passages
Not only can birds not see glass, but large transparent expanses can also have a disorienting effect. When a bird tries to fly through a passage with sheets of invisible glass on one or both sides, this can confuse their senses.
The transparent barriers hinder their ability to stabilize and cause them to repeatedly crash as they struggle to pass through. This behavior has been observed especially around glass walkways, greenhouses, and atria.
Attraction to Reflections
Sometimes it’s not just the illusion of a clear pathway that draws birds to windows, but actual attraction to the reflections. Many species are territorial and instinctively attack their own reflections to drive the “other bird” away.
Some may also see the reflection of plant life and approach thinking they have found a good food source. These types of reflections that actively entice birds can lead to repeated pecking and collisions.
Disorientation from Lighting
Indoor lighting, especially bright or frequently flickering lights, can also confuse birds at night. Nocturnal migrators like songbirds rely on moon, stars, and magnetic fields to navigate. But artificial light shining through windows can cause them to become drawn in and trapped.
As they fly near light sources at night, they may become stunned and repeatedly fly into the windows. Daytime birds may also get disoriented by light reflections on glass hitting their eyes.
Inability to Judge Speed and Distance
Birds have difficulty judging the speed and distance of transparent objects like windows. Most birds fly at speeds of 20 to 40 mph. At that velocity, even a small miscalculation in direction or a delayed response to a window can result in a powerful collision.
Young birds and inexperienced juveniles in particular have the most trouble gauging the presence of glass during high velocity flights. This helps explain why window strikes are one of the top causes of bird deaths worldwide.
Confusion from House Plants
Another reflection that birds may find attractive and inviting is that of house plants. Windows near indoor plants essentially create the illusion of a green corridor or additional vegetation that certain bird species naturally want to fly towards.
Feeders, birdbaths, and other backyard attractants near windows can similarly lure birds towards reflections. This is why recommendations include moving plants at least 3 feet away from windows.
The Urban Environment
There are thought to be up to 1 billion bird deaths each year in the US caused by window collisions. A big reason is because birds in cities and suburbs face much higher concentrations of reflective glass windows than their natural habitats. Glass buildings and homes juxtaposed with trees become death traps.
In addition to glass panes,birds can also fatally collide with other reflective urban objects like metal panels or vehicle surfaces. But windows at homes, offices, and other buildings account for most incidents by far.
High Bird Traffic Areas
Certain areas naturally have higher volumes of bird traffic, making window strikes more likely. These include sites along migratory flyways, bird feeding areas, habitats with limited space, areas near water sources, and regions where birds congregate such as wetlands.
Residences near these hotspots will generally experience more bird-glass collisions. Screening vegetation, marking windows, or other deterrents can help reduce collisions.
Seasonal Factors
There are certain times of year when window collisions increase. Peak seasons are during spring and fall migration periods, when huge numbers of birds are moving through areas. Young birds leaving their nests for the first time are also very vulnerable.
Additionally, seasonal growing cycles affect the types of reflections that can attract birds. New vegetation, ripe fruits and insects on plants near windows, and other seasonal enticements can inadvertently pull birds towards glass surfaces at certain times of the year.
Poor Weather
Inclement weather is another major contributor to window collisions. Low cloud ceilings cause birds to fly at lower altitudes, right in the strike zone of most residences and low-rise buildings. Rainy, stormy, or windy conditions can also hamper birds’ mobility and reaction times.
Reduced visibility, slippery surfaces, and rapid descents to escape the weather put birds at much higher risk of crashing into glass they don’t see. Many collision deaths result from storms and other bad weather.
Residential Building Design
Today’s architectural preferences for more glass, multi-story entries, walk-out basements, and open floor plans increase window collision risks exponentially. Large plate glass windows, wrap-around glass walls, skylights, and glass corners are especially hazardous.
Home features like sunrooms, solariums, glass block walls, and even glass garage doors all can endanger birds. Yet builders rarely use bird-safe practices such as angles, screens, films, patterns, and other deterrents to help prevent crashes.
Window Type
Larger panes are more dangerous than smaller divided panes, as birds can see patterns and objects such as mullions more easily. Modern, energy-efficient windows allowing for greater outside visibility are more reflective and prone to transmitting outside images “through” the glass. Thus, collisions increase.
Low-E (low emissivity) coating further hides visible cues while enhancing mirror effects. Humans can still see UV light markings on treated glass, but most birds cannot. All of these improvements for human aesthetics and efficiency also improve conditions for bird collisions.
Window Position
The position and elevation of windows also plays a role in collision rates. Windows installed higher above ground level receive more impact. This is due to greater acceleration of birds in flight the higher they are off the ground when they change direction. Side, corner, and passageway windows tend to have more crashes as well.
Feeders Near Windows
Well-meaning homeowners unfortunately increase window crashes by putting up bird feeders in their yards. While feeders benefit birds in many ways, placing them too close to windows leads to higher collision mortality.
Birds accelerate rapidly when exiting feeders to escape perceived threats. They also concentrate in higher numbers around feeders, increasing the odds of an accidental crash with reflections they don’t see in time.
More Natural Light
In addition to visibility and aesthetics, another reason modern architecture uses more glass is to allow in more natural light. Floor to ceiling windows and skylights certainly achieve this goal indoors. But outdoors, all that light reflecting off the glass disorients birds.
So in trying to bring the outdoors in through glass, we end up making the outdoors more dangerous for birds. Unobstructed windows allow deadly amounts of light passage for these exterior glass walls and internal reflections.
No Recourse from Glass
Unlike other obstacles birds navigate, like branches or towers, glass offers no “give.” When a bird crashes into a tree or the side of a building, it can recover. But glass provides no such recourse. The bird impacts with full force and has no brush-off opportunity.
This collision energy combined with the stunning effects of transparency explain why window strikes result in fatalities up to 95% of the time. The ultra-hard surfaces used in today’s glass manufacture worsen these already deadly conditions.
Previous Crashes
Evidence shows that birds crashing into windows initially will return and hit the same glass repeatedly. Researchers using test windows observed birds flying into the same place on the glass over 20 times in just 2 hours. Other studies documented up to 18 repeated strikes within 23 minutes.
This tendency of stunned birds to return soon after the first impact adds to the snowball effect. It helps account for the alarming number of bird-glass collisions concentrated in certain windows and homes.
Human Encroachment
As human habitats expand into former wilderness, we increase the interface of buildings and glass within natural bird environments. Birds evolved to navigate forests and fields, not mazes of glass windows and walls in their flight paths.
Urbanization and development place higher volumes of birds in closer proximity to deceptive transparent barriers and reflections. More window crashes result as birds struggle to adapt to built-up areas full of mirrors and glass.
Increased Protection Sought
Growing awareness of worsening bird-glass collision problems has led to some movement to enact legislation for mandatory bird-safe building design. While mainly limited to guidelines so far, continued grassroots efforts and advocacy could lead to future laws requiring bird deterrent glass.
Some options like screens, shutters, films, tape, decals, and angled panes can help. But convincing architects and builders to incorporate truly effective bird-safe practices remains an uphill challenge.
| Season | Factors Increasing Window Collisions |
|---|---|
| Spring | More birds migrating Breeding and nesting behaviors Offspring fledging from nests |
| Summer | Higher vegetation reflections More insects attracted to windows |
| Fall | Migrating birds heading south Frenzied feeding behaviors before migration Young birds migrating for the first time |
| Winter | Window reflections of food sources Disorientation in bad weather Reduced food supplies increasing window attraction |
Conclusion
Bird-window collisions result from a complex interplay of factors. While some causes like weather are unavoidable, being aware of conditions that increase risks allows homeowners to take action. Preventative measures to deter window strikes and reduce mortality can help counteract the inherent dangers reflective glass poses to birds.
With conscious design and proper precautions, we can enjoy the benefits of glass in architecture while also minimizing harm to the birds passing through our environments. Through greater understanding of avian vision, perception, and behaviors around transparent barriers, we can find ways to protect our valued songbirds, migrants, and backyard visitors from deadly window crashes.
