How To Prevent Birds From Crashing Into Windows

Millions of wild birds die each year after hitting the windows. Is there any reliable low-cost way how to prevent them from doing it? Even though I am not a birding expert (yet I build couple of feeders with nice little roofs on them), the methods are quite clear so let’s analyze and compare them.
Why birds often hit the windows
When figuring out the reason why birds often fly into windows we must accept one important fact and that is that people do not see things the same way as birds.

1.) Some birds have their eyes placed in the front of their head. That gives them vision very similar to people. This type of vision is called binocular vision which means that the fields the eyes see do overlap and certain things are perceived with both exes at the same time. Such birds (mainly predators like owls, eagles) have good perception of the distance. The most of non-predatory birds have their eyes placed on the sides of their head. The fields of vision do not overlap (monocular vision) which gives them much wider view but reduces their ability to perceive the distance from the objects in front of them.

2.) Birds are not people, have not invented the glass and do not know what windows are (OK some dirty ole parrots know them very well but the majority of wild birds don’t). They can perceive the glass of the window in 3 ways (A, B, C).
(A) When there are no reflections on the glass, it can appear to them transparent. Birds simply see what’s in the room and if it doesn’t threaten them they may want to fly into the room but hit the window instead.
(B) They can see the window semitransparent which means that they see both the interior of the room and some exterior reflections.
(C) They see the reflections in the window, e.g. they can see themselves, considering their own reflection an intruder and may attack it, hitting the hard glass.

3.)The birds can see things beyond human perception. There’s nothing mysterious with it, simply, the most of the birds see also in ultraviolet (UV) spectrum of the light, while people do not perceive it. That means that birds can see the reflections on the window even in situations when people would not see them.
Ways to stop the birds from flying into windows
Tip #1 - never wash your windows
No, I am not serious, of course you should wash your windows but - definitely - this would work. Dirty enough windows would be perceived solid objects by birds just like the walls. No insight, no reflections, no dead birds.
Price: $0
Recommended: no, this just was a joke

Tip # 2 - the stick-on birds
You can buy paper silhouettes of a predatory birds (eagle, hawk) and stick them on the window. You don’t even have to buy them. Just draw the silhouette (based on some picture from book) on the paper, cut it with the scissors and stick it on the window. The silhouette must be realistic, best if it shows the predator in hunting posture. The problem is that the birds get used to it over time. For example, I live near the large bakery. For years, this bakery has problems with birds trying to feed on the grain and flour they stock. Of course, they tried everything to prevent the birds from flying in the factory, they also tried the stick-on birds. Nice, professional pictures in realistic postures and dimension. Worked for limited time. After some time, the birds were flying in and out the factory again, regardless of the eagle stickers. Now they have window screens everywhere.
Price: $0
Recommended: yes, you should try it at least. You can make them yourself, maybe with a little help of your children (they will like it, it will be a mission of the day for the little guys) at zero cost. If it stops working for you, nothing happens.

Tip # 3 - the window screens
The window screens will not prevent birds from flying into the windows (though they may reduce the occurrence) but will definitely protect both bird and the window during the crash.
I suppose that majority of people would feel sad seeing birds dying at their window sill. I would for sure and if that happened often I would try to find out how to protect those birds. If the window is covered with the screen (typically an insect screen made of thin metal wire, fiberglass or synthetic fiber) and a bird flies into it, the crash is much softer and the bird - though shaken - recovers from it almost immediately without any injuries. The window mesh also protects your home from insects.
Price: depends on dimensions but typically $10-$50/window
Recommended: yes, even though occasional crashes with birds must be still expected, the window mesh will protect both window and bird

Tip # 4 - ultraviolet window decals
Like the paper hawk stickers, these decals are also to be sticked on the window, the difference is that they are transparent for people (so that you can still enjoy the view from the window) but opaque for birds. This is achieved by special layer of the decal which reflects the ultraviolet (UV) light making it visible for the birds. The window decals are not meant to threat the birds (like the paper predator stickers) but rather to create a virtual wall visible on for them therefore one decal is not enough and you will have to buy them many and literally wallpaper the window with them which makes this method little too expensive.
Price: usually $100+/window
Recommended: yes, even though occasional crashes with birds are still possible
So what is the best method to keep birds from crashing into windows?
In my opinion it is the window screens. At medium cost you will get reliable shield protecting both bird and window. Currently there are many window screens available on the market that last quite long time (e.g. vinyl covered fiberglass screens), their installation is easy and require almost no maintenance (shower them twice a year in the bathtub, that’s all).