Take on the AC grate

There is one phenomenon which I just have not been able to figure out.
You walk down any street in Brooklyn and you see thousands of window
unit AC's sticking out of the buildings and ever single one of them is
damage in the back. I can't seem to figure out if they come
pre-damaged or if there are a bunch of idiot pigeons crashing into
these things but the are all damaged in the exact same way so unless
we blame it on the birds I am convinced it's part of an alien
invasion.