Wondrous item, common
When a bird carrying a written message comes within 1 mile of the clock and is flying toward it, the small door opens and the horn bird comes out on its wire. The door opens as long before the bird's arrival as the bird needs to cover the ground, up to 1 hour, and stays open until the bird arrives or turns aside. The bird on the wire points the quarter the message is coming from, not the degree.
It does not open for a letter carried by a person, or by a beast that walks. It has never opened for a message that was not written down.
Source: Provenance — The Quist Sale