Birds' eyes

http://www.earthlife.net/birds/vision.html

It says:
"In man, binocular vision is about 140 degrees out of a total of about 180 degrees. In a pigeon though the binocular area is only 20-30 degrees out of a total field of vision of 300-340 degrees. In many raptors and owls the situation is different. In these birds, as in many insectivorous birds, binocular vision, important in making judgements of distance, is more necessary and so these birds have their eyes more towards the front of their heads. This is most evident in owls where the total field of view is reduced to about 110 degrees with a binocular vision of 70 degrees. This is why owls turn their heads to watch you walk past. An owl can turn its head through over 200 degrees but cannot move its eyes in its head at all."

Maybe the lack of binocular vision makes littler birds more likely to fail to see glass for what it is.