The following is from the NPR site that Desert Dancer provided:
Morning Edition, January 3, 2006. No one knows what birds see when they look out at the world, says ornithologist Daniel Klem, but he's sure they don't see glass. He estimates that at least 1 billion birds are killed by flying into windows every year in the United States. LINK
Compare that with, "Experts say outdoor cats may killhundreds of millions of wild birds each year -- but they aren't exactly sure how many." From the same site. (Emphasis added by rags).
My own experience was, that in the 18 years that my last cat lived, and in the ten years of his twin's life, in spite of all attempts I made to keep birds away from them, my windows killed many times more birds than my two cats ever did. The cats are long gone. The windows continue to kill.