I would NEVER post a sign letting anyone who comes to my door that I have birds inside.... I keep my front door locked, when the door bell rings, if I don't know who it is, I either don't answer it, or I quickly step outside and close the door behind me. If I know who it is, I open the door about 2 inches and tell them to hurry in as I have birds flying around my house, and then I close the door and check to see where my birds are, if they are close by, I tell them to wait while I put my birds in a safe spot or have my kids watch them.
People have broken into homes to steal their expensive birds....that won't happen on my watch! I even have a house alarm.