tenitztli.

TEN-ITZ-TLI, literally, “obsidian bill,” possibly the Black Tern (Chlidonias niger) [FC: 31 Tenitzli] “It flies high always at night there over the lagoon {Lago Texcoco}. It is the same size as a dove. Its head is quite small, black. Its breast is somewhat white, somewhat dark. Its back is black; its wings quite small. Its body is all small and round, its tail small, and its legs are like a dove’s. For this reason it is called “obsidian bill”: it has three bills in all. Its food enters in two places, [though] there is only one throat by which it swallows it. It has also two tongues. Its [three] bills are one over the other…. The food of the is water flies, flying ants which fly high.” Martin del Campo suggested that this strange bird might be the Black Skimmer (Rhynchops niger) because of the unique shape of that bird’s beak. However, that is a most unlikely identification. The skimmer is a large, long-winged bird of the ocean shore that only rarely might stray to the highlands. It seems more likely that this could refer to the Black Tern (Chlidonias niger).
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 11 – Earthly Things, no. 14, Part XII, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1963); Rafael Martín del Campo, “Ensayo de interpretación del Libro Undecimo de la Historia General de las Cosas de Nueva España de Fray Bernardino de Sahagún – 11 Las Aves (1),” Anales del Instituto de Biología Tomo XI, Núm. 1 (México, D.F., 1940); and, with quotation selections, synthesis, and analysis here also appearing in E. S. Hunn, "The Aztec Fascination with Birds: Deciphering Sixteenth-Century Sources," unpublished manuscript, 2022, cited here with permission.

The Getty's Digital Florentine Codex includes an image of the tenitztli bird:
https://florentinecodex.getty.edu/book/11/folio/31v/images/c24e8d5d-93c2....