Principal English Translation: 

Least Bittern (bird) (see Hunn, in attestations)

Attestations from sources in English: 

Ā-CACHICHIC-TLI, Least Bittern (Ixobrychus exilis) [FC: 39 Acachichictli] “It lives on the water. It is named acachichictli because it is as if it sings achichichic. And it lives only among the canes, the reeds…. Its head is quite small; its bill is pointed and small. All of its feathers are yellow, slightly ashen. Its legs are yellow, greenish. It always lives here; it rears its young here. Four are its eggs;…” Martin del Campo suggested Aechmophorus occidentalis (now Western and/or Clark’s Grebe). A quite different name seems well established for the Western/Clark’s Grebe, Ā-CI-TLI [FC: 31]. Also, these grebes do not have “yellow feathers” or “yellow feet,” nor are the typical vocalizations of these grebes similar to the calls described. I believe the proper referent is most likely the Least Bittern (Ixobrychus exilis), which is resident to 2500 meters elevation in Central Mexican marshlands, favors dense reed beds for nesting and foraging, and fits the description nicely, including the calls as rendered in this onomatopoetic name.
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analysis here also appearing in E. S. Hunn, "The Aztec Fascination with Birds: Deciphering Sixteenth-Century Sources," unpublished manuscript, 2022, cited here with permission.