"The drum-major headdress is part of a specific costume created by Bird Jaguar IV. The headdress appears only on lintels. It was first portrayed on the lintels of the three structures Bird Jaguar IV dedicated in 126.96.36.199.0. The tall cylindrical headdress sits on the head of a large bird or other animal with a down-turned, hooked beak. This is the headdress Bird Jaguar IV wore on his accession lintel (1), and it is the headdress most frequently associated with the holding of the God K manikin scepter and the bundle. It is part of the regalia for dynastic succession and accession anniversaries. When Bird Jaguar IV wears this headdress, he always wears a backrack.
This headdress is a local variation of the drum-major headdress worn in accessions at Palenque, where it was handed to Pacal by his mother as depicted on that site's earliest figural monument, the Oval Palace Tablet, dating to after 188.8.131.52.8 (A.D. 615). At Yaxchilan it was adopted during the reign of Bird Jaguar IV in an obvious attempt to participate in a regional (Western Maya) tradition."
Carolyn E. Tate, Yaxchilan: The Design of a Maya Ceremonial City, p. 70