Birds fly against the window (Garage window)

Use knitting yarn to make parallel or horizontal lines on the windows. Affix on the ends with invisible scotch tape. 

For attic and/or garage windows this method could be something to try because it wouldn't matter if the yarn was obvious. For a living room or office window the yarn might be too obvious and more of an annoyance.

There are special reflective decals that only birds are able to see and there is also standard mylar tape and stickers from any craft store that can be purchased and applied to the interior of the windows. 

Amazon carries a good selection including a wide variety from the top-rated Window Alert brand. For example check out this one.

Bird Protection Glass is glass with a UV-reflective coating. It isn't visible to people but is visible to birds. 

Ornilux is a brand that is well-known for their bird protection glass and is what the Philadelphia Zoo uses as their primary method of stopping birds from fatally colliding with their windows. 

This solution is more sensible for commercial businesses and office buildings than residential homes due to the costs. However, once installed it is a very effective solution! 

There are 2 main reasons why birds fly into glass:

1. Either they don't see the window as a solid barrier and see the reflection as open sky OR 

2. They see the reflection of themselves and want to attack the intruder. 

Both reasons involve the window's reflection. Therefore, breaking the reflection significantly reduces the risk of bird-window collissions. 

One way to break the reflection is to take a bar of soap and draw streaks across the window. This would be a good option to try during spring when collissions occur more frequently as birds become more territorial towards their breeding grounds. 

The soap streaks will need to be checked every couple days and reapplied due to soap washing away when it rains or just fading and becoming less visible.

Attach a window screen &/or a mesh-like netting a couple inches away from the outside of the window. This will stop the birds prior to impact and will safely bounce them off. 

Home Depot, Lowes, and other home hardware stores carry rolls of special protective mesh that is cheap and effective! 

Putting exterior shutters on the windows would keep birds from seeing the reflection of the sky and believing it was a safe place to fly and from seeing a reflection of themselves and launching a territorial attack on the intruding bird. 

This method is obviously only effective when the shutters are closed and having them closed would mean having a room with little to no natural light. If this isn't a high-traffic room (i.e. mud room, garage, attic, etc.) it would probably be a better option than a family room or bedroom.

There is a special film that is applied to the window that is invisible to those inside but from the outside is opaque. 

Amazon carries several variations of window film including Bird's Eye View by Artscape. This film is applied on the interior of the window. For example check out this one.

Another film that has many options available (i.e. tinted, pattern/decor, white, clear, etc.)  and is also applied to the interior of the window is CollideEscape which can be found here.

Keep interior blinds and shades partially closed. This will allow birds to see contrast and realize that the window is not open sky/part of the environment. 

This option would definitely be for those that leave the house during the day and/or don't need the shades open.

Use tempera paint to create a design on the windows. It can be a simple stencil pattern or an actual painting with a subject and background. 

The average homeowner and commercial business (i.e. bank, clothing store, restaurant, etc.) would probably not want to paint on their windows, however, for childcare centers/schools, art studios, bookstores---this would be something a little more suitable.

There are birds that will see the reflection in a window and believe it to be a bird competing for a mate and/or for territory (and therefore attack the "intruder"). These birds are called territorial birds.

There is a special temporary spray that can be used safely on windows that eliminates reflection and thereby stops the bird from harming and/or potentially killing itself. The spray aims to be invisible however some product reviewers claim it looks like graffiti on their windows and disappointed to have their views obscured. Check it out here.

In any case, it seems to be only a temporary solution and will have to be reapplied as soon as it starts to fade.