I'm frustrated coz I have this old neighbor who lives like 2 floors above me.... and since 2 years he has this new hobby (being retired) - he became a pigeon fancier

Our complex of apartments is pretty neat and clean and all the neighbors are the same. He was too, until he retired like 2 years ago.
Thing is I don't really care what he does, but all these pigeons he "tamed" sometimes are landing on my external windowsill too and it's not a pleasant thing to see them
shi........ng all over it.
I talked with him a couple of times, but it seems he doesn't understand.....
We also have these mailboxes on the ground floor all pinned into this huge wall and when they bring our correspondence or bills they put them inside, but because the mailboxes are not so huge sometimes the envelopes are hanging out of there. So it is kinda easy to take anyone's envelope/bill out of their mailbox.
Of course until now no one did anything and we all receive on a monthly basis our correspondence because we respect our neighbors (so far)....
I'm thinking - because I tried everything '- to make his bills / correspondence disappear

But I ain't sure yet.....
Any suggestions ? About how could I make this guy quit his hobby ?