This, you see, makes it even more complicated than it already is. In order for this image/idea to be so, his parents would had to have seen him as a bird to begin with... and if that were the case, they certainly wouldn’t have left the window open. It also implies that he mutates fully into a human later on in life. If THAT were the case, then he’d lose his special magical quality of being neither-nor. Besides all the [warped] logistics of it contained therein and beyond, we can’t ignore the fact that Barrie had been well aware of Rackham’s drawings. Which means that if Pan had been meant to literally be “betwixt-and-between” in physical form, Barrie would have corrected Rackham’s interpretation. Sure he would have, given how fussy and attentive he’d been when it came to his eternal boy.
For anyone needing clarification on "Betwixt-and-Between" you'll certainly find it in my interquel novel. As you might expect, it expands the idea into the broader scope of the character of Peter Pan.