Pigeons, the infamous flying rats… 

It's really not nice to discover them starting to pile up tiny wood twigs on your BED because they were able to enter through the half open window and shades on a spring sunny day.
These devils carry fleas and all sort of pests.

Yes, it happened to me. Never again !

Here is a shade blocker to allow venting the room but not allow pigeons in.

Of course it's totally customized to one kind of crappy old shades, but I think they might be common enough in France, so, just in case this may be useful…

I printed this at 0.3 with PLA