Some estimates (seen in my five seconds of google) place the annual number of bird deaths in the USA due to glass collisions at over 1 billion. So, globably speaking, what? 10 billion-ish birds die from glass collision? More? The global population of birds according to one paper I found is 200-400 birds. Rough,rough guesstimate, but that sounds absurdly high, especially considering most of those birds wouldn't be anywhere near glass. Even in just a few generations, a death rate that high would surely start to affect the populations somehow.