Each year somewhere between 100 million and 1 billion birds are estimated to hit and be killed by glass in the US alone.
Although the BBC argues that there’s no concrete evidence for these numbers, almost all of us are familiar with the horrifying sound of a bird flying into a window.
Because of the extensive range of colors observed by birds, as well as their wide field of view, birds can’t see regular glass.
Below we will take a deeper look into how birds’ eyes work, why they can’t see glass, why they fly into windows, and what you can do to prevent it from happening in the future.
- 1 A Quick Look At How Birds See
- 2 Why Can’t Birds See Glass?
- 3 Why Do Birds Hit Windows?
- 4 Why Do Birds Attack Glass?
- 5 How to Prevent Birds From Hitting Glass
A Quick Look At How Birds See
Similar to our eyes, birds’ eyes are made up of lenses, corneas, and retinas, however, it may be a surprise that their eyesight far surpasses that of a human.
Although eyesight varies between species, birds’ eyes are all made up of cones and rods.
In short, rods allow for some birds to have better night vision, but a less sharp image, while cones sharpen images, but make the eyes less sensitive to light.
Some birds such as hawks have as many as 1,000,000 cones per square millimeter, that’s nearly 3 times that of a human.
This allows them to see in fine detail with high definition.
Although there are thousands of individual cones, most birds’ eyes are made up of five types of cones.
Four are dedicated to tetrachromatic color vision and the fifth to achromatic motion perception.
The four color-orientated cones are what give birds’ eyes the ability to see more colors than humans can.
Thanks to the additional cones (humans only have three of them, red, green, and blue), birds are able to see ultraviolet light.
As can be seen in the image above, humans can see a light spectrum of between 380 and 700 nanometers.
Thanks to the additional cone in their eyes, birds’ vision extends between 300 and 700 nanometers, thus introducing the UV spectrum to their vision.
So what do we know about birds’ eyes?
1. They see in more detail than humans.
2. Birds can see UV light.
But how does this relate to glass?
Why Can’t Birds See Glass?
Glass, particularly the ones we use on our windows, is made to be as invisible as possible, and in some cases is not visible to humans, let alone other animals.
Most standard window glass, according to the International Ultraviolet Association, will filter out almost all UV-B and UV-C light.
As birds see using UV light, with these colors removed from the glass, the surface becomes invisible to the bird.
Furthermore, thanks to their high-definition sight, images such as reflections appear brighter and in more detail.
If a window were to reflect the surrounding area such as bushes, the sky, and some trees, we may see a reflection, but birds may just view a continuing landscape.
Why Do Birds Hit Windows?
Birds can’t see glass, and for the most part, neither can humans. Although humans do walk into glass doors and windows from time to time, it’s nowhere near as much as birds.
So why do birds hit windows so much more than humans and other animals?
1. Birds Don’t Understand Glass
As a child, you likely walked into a window at least once, but with constant reminders from your parents and a brain that can work around the concept, we begin to understand glass.
Once we understand what glass is, we are much more aware of it, and therefore (when we’re conscious) avoid colliding with it.
Birds, on the other hand, can’t understand the concept of glass, partly due to their inability to see it, and partly due to the different functioning of the brain. This means that a bird is never on the lookout for glass, nor will it understand why the sky suddenly became hard.
This is why a bird trapped in a house will continually fly against a window, even to their own demise.
2. Birds See More Intense Color
As mentioned before, because birds not only see more color but with more detail, reflections on the window are much clearer.
Consider a tall building covered in large glass windows. The reflection of the sky and clouds can make the building near invisible, even to the human eye.
To a bird, it may not seem to exist at all. Similarly, garden windows often reflect trees and other surrounding plants, giving birds the impression that there is foliage to continue into.
3. Flying in Panic or Fear
Birds that hit windows are often found to have been startled or flying in fear of a predator. Similarly to humans, when birds are frightened, they lose focus and adopt a “flee” mentality.
Although in open space this can be the difference between life or death, with invisible windows around, birds are more likely to fly into them at full speed.
4. Attempting to Reach the Other Side
It’s believed that a major reason for birds to hit windows, particularly those found on the upper levels of buildings, is that they create an illusion of a tunnel running through.
In buildings with large opposite windows, birds could perceive this as a tunnel to get through the building. As they can’t see, nor comprise the windows, collisions become common.
Why Do Birds Attack Glass?
Not quite like collisions, but with the same concept, it’s common to see birds attacking their reflection in a mirror or window.
This is sometimes done to such an extent that the bird can injure itself.
This type of behavior is most common during the breeding season when aggressive male birds attempt to defend their females, nests, and food sources from encroaching males.
Because of the high-quality detail and their lack of understanding of reflections, birds can easily mistake their reflection for another bird.
How to Prevent Birds From Hitting Glass
As birds can’t see windows, but we want to be able to see outside, it becomes our responsibility to take action toward preventing collisions.
This can be done through various means.
1. Make Use of Bird-Safe Glass
Bird-friendly glass is specifically designed to make the glass visible to birds while maintaining its transparent look to humans.
This is done in a variety of ways such as fritting, silk-screening, or by using an ultraviolet coating which creates a pattern that breaks up the reflectivity of the glass.
Although various techniques are used, studies have shown that the most important is by spacing the coating or markings two inches apart horizontally and four inches apart vertically.
This is known as the “2×4 rule”.
2. Use Bird Tape
Bird tape is a simple, long-lasting solution to preventing birds from hitting your windows if you can’t afford to replace them all with bird-friendly glass.
Bird tape makes correct spacing simple while adding a shimmer in the 2×4 pattern.
Although the tape will still slightly disrupt your view (it will seem as though you are looking through a screen door), the light will still pass through the tape and windows.
This is the cheapest and simplest solution to protect your local birds from your deadly windows.
3. Make Use of Tape strips or Collision Stickers
Using tape strips or collision stickers can work, but they are nowhere near as effective as the previously mentioned methods, nor are they as easy to apply.
Although tape is applied in mostly the same fashion, because it’s difficult to replicate the 2×4 rule, using random tape may not work as expected, however, it is better than nothing.
To apply the tape, when using white tape, cut 1/4” strips, or with black tape use 1/8” strips.
White tape should be placed 4” apart, vertically, while black strips should be placed in a horizontal layout, spaced 1” apart.
4. Use Mosquito Screens or Blinds
Mosquito screens and blinds are great as they both serve multiple purposes.
Not only will blinds keep your house cool in summer and prevent any peeping toms, but they will create enough of an image behind the glass for birds to notice.
5. Draw Your Curtains
Drawing your curtains, particularly at night, can prevent birds from colliding with windows, as it will seem to them as if there is a solid object in the way.
Nocturnal birds are prone to hitting windows with lights behind them as the artificial light distracts them from their normal state.
By drawing your curtains you can block out some of this light and prevent night birds from attempting to fly through.
5. Try A Feather Distraction
Because birds often hit windows due to being startled and a lack of attention, one thing we can do to prevent them from hitting windows is by grabbing that attention. This can be done by setting up objects that act as a sort of warning.
To a bird, an environment with a large number of loose feathers could indicate that a predator is near, and therefore sends a message that the bird should avoid the area.
By attacking feathers to clear cords such as fishing lines, and hanging them vertically in front of windows that are commonly collided with, you may be able to prevent future incidents.