I'm taking a shot in the dark here, but I wonder if that's because of the clear glass vs. "frosted" plastic. Even though they don't rely on their eyesight as much as some other senses, maybe it's enough for them to tell a difference between the two and since BP's typically like to "hide", it makes them feel more threatened, so they are more aggressive? I know they say to cover 3 of the 4 sides when they are babies a lot of the time to help them be less stressed. I know this may be totally off base, but it's a thought.