We just finished building our coop today and put in a glass window. So now I'm wondering about this too. I hope it's ok.
I have a big glass window in mine and they just love to sit in front of it and bath in the sunlight. Plus it lets in light,,,it gets dark in there when gathering the eggs,,its nice to be able to see.
I have plexi-glass in mine and it faces south. I may be crazy but it gives me eggs earlier in the year because they get more light in the coop.
I have two glass windows and it's fine. It's best though to install the windows backwards so you can open and close them from outside the coop- comes in handy.
We have a glass window that slides open and closed. I was worried they would try to fly through the glass so we put a couple pieces of tape across the window thinking it would help them see the glass when the window was closed. I get the feeling now that they didn't really need the tape, they see the glass just fine. So far, so good.
