As already pointed out, birds are unable to see glass and the concept of something being transparent and solid at the same time is something they don't know. The only transparent medium they usually encounter is air.
While in hot persuit of an insect, a bird can sometimes hit a window even if you have curtains or blinds. As they don't see the pane, they believe they have enough space left to maneuvre.
Especially small birds often try to pass through the gaps in blinds as if they were twigs.
These bird-of-prey silhouettes we Germans use on public buildings and other big glass panes have over the years proved to not work as well as we had hoped. Some birds believe where another bird "flies" they can fly as well and crash into the pane. Others don't recognize the birds of prey as such (a falcon and a dove are surprisingly similar if viewed from below). Others again ignore the silhouettes because they are the wrong species of predator (a species that hunts only doves will be ignored by sparrows).
There is a new invention, however, which looks quite promising. It's called birdpen. You use it to draw a crisscross pattern (or whatever you like) on the window pane. The pen's ink is invisible for humans (sometimes, at the right angle you can see a slight blur) but it reflects UV light and therefore is very visible to birds. This way they can see the obstacle and stop flying into it.
I just had a look and it appears that neither the silhouettes nor the birdpen are available at Amazon.com, but you can get the silhouettes at amazon.co.uk (British) and the birdpen at Amazon.de (German).
(currently out of stock)
If you need help with ordering in German, I'll gladly help you as soon as it's in stock again.