I have had the problem with birds building nests under my window AC units.
They can get in and out due to the large openings around the storm window where the AC passes through.
My solution so far has been stuffing window screening (the old, steel type) into the opening. That seems to work pretty well, but is a hassle to install.
I'm wondering whether there is any product that would fit onto the AC unit and close off the opening, while at the same time allow the air to circulate around the unit's vents.
Unfortunately, I cannot get access to the window units once they are installed, as they are on the 2nd floor.
If I could fit something onto the AC before sliding it into the window, I think this would do a better job, and be much easier to use/install.