Birds have exceptionally good vision. Birds of prey such as eagles and falcons can see eight times further than humans can, and owls have a specialised lens at the back of their eyes to help them see at night. Despite eyesight that is generally superior to humans, birds are often injured or killed by flying into glass, which begs the question, can birds see glass?
Birds don’t learn the visual cues that inform their brain that an object is glass - glass is a human invention, after all. Bird vision is also fundamentally different to ours - it’s incredibly sharp, but they don’t perceive depth in the same way as mammals. Furthermore, birds misunderstand reflections in glass such as the sky and trees, mistaking them for the real thing.
In fact, most animals won’t understand glass until they’re given a chance to learn, the same as humans - we are not born with perception and understanding of glass. Modern glass is also exceptionally clear, and even humans can walk into it from time to time, so it’s certainly no wonder that birds make the same mistake when flying at high velocity.
Read on to discover more interesting facts about how birds perceive glass.
Bird looking at reflection in the window
Most birds seem hyper-aware of their surroundings, hence why we often have to keep a suitable distance when observing them in the wild. If you get too close to a bird, their sharp eyes will spot you, and they’ll fly off. Next time you see a bird, observe how large its eyes are relative to its body, and therefore to its brain - this is partly why bird vision is so sharp. Not only do birds have large eyes, but their eyes are densely packed with receptors to enhance visual acuity and other adaptations like extra lenses to reflect more light.
Despite this, millions of birds are killed flying into windows every year, so it’s a pertinent question to consider why birds are clumsy when it comes to windows and glass.
There are several explanations. Firstly, bird eyes are generally mounted towards the side of their heads to maximise their field of vision, but this comes at the expense of depth perception.
Contrastingly, human eyes point straight forward, which means each eye’s field of vision overlaps, which enhances depth perception at the expense of our field of vision. This means that birds do not perhaps perceive the depth of 3D structures that contain glass, like buildings.
Birds also see and perceive a wider depth of colours than humans and see colours more intensely too. Birds often confuse their reflection for another bird, likely because they perceive the reflected colours strongly enough to mistake it for being real. As such, when birds see reflections of the sky, trees, etc, in windows and glass, it probably looks realistic to them.
Perhaps the most important factor is that birds do not learn about glass in their natural environment - it’s not part of their ‘curriculum’, so to speak. Birds and other animals will learn to perceive glass if given a chance.
A blue tit fighting its own reflection in a window
Glass manufacturers now make bird-safe glass which is being used on the outside of tower blocks and other tall structures to prevent bird fatalities and damage to the glass. Bird-safe glass enhances the reflection of UV that birds see so intensely, thus helping birds perceive the glass as a barrier and not as simply a continuation of the environment.
DIY methods include blacking out the windows whenever possible, moving plants away from windows and adding large decals or posters to the window. You can also purchase ‘birdsavers’ which are specialist films designed to be laid over the top of windows - they’re popular for new eco-friendly house builds and have other benefits like filtering excessive infrared light.
Zen curtains are another DIY method that uses pieces of string laid across a window to signal birds that the window is non-traversable.
If you see or hear a bird strike your window, then check nearby to see if the bird is laid on the ground. It’s quite possible that they’ll fly away immediately if they’re relatively unharmed.
If the bird is laid motionless on the ground, then it may be stunned, injured or dead. Firstly, check for obvious injuries such as cuts or breakages. If you suspect a broken wing, leg, or another injured body part, then contact your local wildlife hospital. If the bird looks uninjured but isn’t moving, then it may be stunned or may have been killed by the impact.
To check whether a bird is stunned or dead, look to see if its beak is open and whether there are signs of breathing. You may also be able to feel its heartbeat if you lay two fingers across its chest. Don’t assume a bird is dead if you can’t sense breathing or feel its heartbeat, as these slow considerably when a bird is stunned. Place it in a box and wait to see if it recovers, or call a wildlife hospital for advice.
Stunned birds are essentially knocked out or concussed, and they need to be given time to safely recover. If you believe the bird is stunned, place it in a warm, dark box with some soft padding. Stunned birds can take 2 to 3 hours to recover.
Starling looking at reflection in window
Given the velocities birds are often travelling when they hit a window, many do not survive the impact.
The exact figure of how many birds die from window collisions each year is unclear. A much-quoted figure is 1 billion, which originates from a 1990 paper written by biologist Professor Daniel Klem in the Journal of Field Ornithology.
The British Trust for Ornithology estimated that some 100 million bird collisions occur in the UK each year and that around a third are likely fatal.
These are just estimates, but they still give a sense of the problem, particularly in cities that intersect the migratory routes of some birds. Spring is regarded as the worst time for bird collisions given migratory behaviours, and tall buildings are by far the worst offenders compared to standard 2-story houses.
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