In the UK the British Trust for Ornithology estimated a few years ago that 100 million birds hit windows each year.
I have tried all the usual ways of trying to stop the birds hitting the windows including sticking hawk shapes in the window, which just don't work.
Eventually I came up with this really cheap and simple solution.
Please share this with someone you know who loves birds. Alex
The gap is probably less than 4" , probably 3 " and the pen I used is like this (but any white "write on glass" pen will do). Make sure it is white as I think part of the reason it works is by triggering bird's fear of flying into cobwebs.