Question

why does the poet write,"it was hawk" rather then "it was the hawk"

Answer 1

Well, usually most poetry doesn't use the word "the" in the poem. The reason is because it might sound too extra.
Hope i got it correct!
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Answer 2

its b/c he is probably not using the work hawk to mean like, a bird. hawk has another definition "a person who aggressive of warlike policy" I'd have to see the sentence that comes before this but the poet is probably referring to something aggressive or warlike.