Carolyn Tate writes that large oval cartouches containing portrait busts of ancestors were used at a number of prominant Maya cities, including Palenque, Tikal and Yaxchilan. Wherever they are found, ancestor cartouches are marked with celestial signs. Here, a bird, possibly a macaw, appears to be rising from the hieroglyph for Venus -- the "V" shaped symbol with two circles underneath. Unfortunately, the ancestor portraits have fallen away and only the frames are still visible.
Carolyn E. Tate, Yaxchilan: The Design of a Maya Ceremonial City, p. 59-61