This gives me the hypothesis that Big Bird is actually a Simurgh, a gargantuan type of mythical bird made up of the unity of thirty birds in enlightenment (At least according to the most famous version of its story spoken of by Borges).

This would explain his great size despite his age and his seemingly-indeterminate species of origin, as well as his golden color relating to Western alchemy’s symbol for enlightenment.

Boy is it fun to overanalyze Sesame Street as a fantasy setting…

[Source is at puppet . wikia . com/wiki/Big_Bird, which I cannot directly link because @staff hates competition.]