Principal English Translation: 

a heron or an egret (both are white birds); also, a heron-feather headdress (see Molina and attestations)

Alonso de Molina: 

aztatl. garza.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 10r. col. 2. Thanks to Joe Campbell for providing the transcription.

Attestations from sources in English: 

Naztauh, nomecaxicol. Quitoznequi: inic onechtequimacac in altepetl: ic in itlacauh oninochiuh intla niquitlacoz, intla itla ic nicouitiliz: nictzactiaz. = My heron-feather headdress, my jacket of ropes. This means: When the city gives me a responsibility I become a slave. If I hurt the city in some way, if I endanger it, I shall be put in jail.
Thelma D. Sullivan, "Nahuatl Proverbs, Conundrums, and Metaphors, Collected by Sahagún," Estudios de Cultura Náhuatl 4 (1963), 140–141.

aztatl = egret (late sixteenth-century, Tetzcoco?)
Ballads of the Lords of New Spain: The Codex Romances de los Señores de la Nueva España, transcribed and translated by John Bierhorst (Austin: University of Texas Press, 2009), 35.

auh yoan yn jquac y, tlaacopiloaia: auh ynaca oc concholotiaia, yn iqueztepol malli: yoan mecaxicolli, aztapalactontli itech pilcac = Moreover, at this same time he put up [on the point of the pole], so that it hung—having removed the [remaining] flesh—the thigh bone of the captive, and suspended with it the sleeveless knotted cord jacket and a small spray of heron feathers. (16th century, Mexico City)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 2—The Ceremonies, No. 14, Part III, 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, 1951), 57.