with the meaning “its real, true, or normal self” [the bird is not itself today]: in this construction, it functions as an adjective and self as a noun; when they are separated, the form its is used: its own sweet self
See itself in American Heritage Dictionary 4
pron.
That one identical with it:
a. Used reflexively as the direct or indirect object of a verb or the object of a preposition: The cat scratched itself.
b. Used for emphasis: The trouble is in the machine itself.
c. Used in an absolute construction: Itself no great poem, it still reveals talent.
Its normal or healthy condition or state: The car is acting itself again since we changed the oil.