A couple of recent articles on NBI from Ancient Egypt, from the UCLA Encyclopedia of Egyptology.
Three different kinds of ibis species are attested from ancient Egypt: the sacred ibis, the glossy ibis, and the northern bald ibis. Pictorial representations of the latter bird—easily recognizable by the shape of its body, the shorter legs, long curved beak, and the typical crest covering the back of the head—were used in writings of the noun akh and related words and notions (e.g., the blessed dead). We can deduce from modern observations that in ancient times this member of the ibis species used to dwell on rocky cliffs on the eastern bank of the Nile, that is, at the very place designated as the ideal rebirth and resurrection region (the akhet). Thus, the northern bald ibises might have been viewed as visitors and messengers from the other world—earthly manifestations of the blessed dead (the akhu).
Janák, Jíří. (2013). Akh. UCLA Encyclopedia of Egyptology, 1(1). nelc_uee_8790.
Three different kinds of ibis species are attested from ancient Egypt: the sacred ibis, the glossy ibis, and the northern bald ibis. Pictorial representations of the latter bird—easily recognizable by the shape of its body, the shorter legs, long curved beak, and the typical crest covering the back of the head—were used in writings of the noun akh and related words and notions (e.g., the blessed dead). We can deduce from modern observations that in ancient times this member of the ibis species used to dwell on rocky cliffs on the eastern bank of the Nile, that is, at the very place designated as the ideal rebirth and resurrection region (the akhet). Thus, the northern bald ibises might have been viewed as visitors and messengers from the other world—earthly manifestations of the blessed dead (the akhu).
Janák, Jíří. (2013). Akh. UCLA Encyclopedia of Egyptology, 1(1). nelc_uee_8790.