To be able to define the narrative hook in James Hurst's short story The Scarlet Ibis, one must first understand what a narrative hook is.
A narrative hook is a technique, used by authors, to insure (or try to insure) they "hook," or gain, the reader's attention. The author uses the hook to insure that the reader continues to read.
Therefore, narrative hooks are subjective. Not all readers are "hooked" by all openings. As hard as some authors try, they can not insure that their opening will grab the attention of all readers.
That being said, the narrative hook (for me) is the opening line:
Summer was dead, but autumn had not yet been born when the ibis came to the bleeding tree.
I find the language beautiful and the imagery fascinating. Both the image of the "dead" summer and the fact that "autumn had not yet been born" is fascinating based upon the question the description leaves in my mind: If summer is dead and autumn has yet to arrive, where are we (in regards to the season)?
From this point on, Hurst's narrative hook has (used in the present tense based upon the fact that it perplexes me every time I read it) me "hooked."
Outside of that, the narrative hook is, or can be, different for each and every reader.