Actually, the crow is the subject!
the structure of the sentence is like this:
In the elm tree (where? we can substitute this phrase with "there")
cawed - this is the verb of the sentence
one large black crow - this is the full subject of the sentence, let's substitute this sentence with "it"
then we have a sentence "there cawed it" - now it's easier to see that the "it" must be a subject.
then in the phrase
one large black crow
the head is Crow - that's the correct answer - and the rest of the words are adjectives to give more information about this Crow.