The poet compared imagination to a soaring bird because imagination is limitless, it can do anything and go anywhere, much like a soaring bird, who has the freedom and capabilites to do anything. Both are completely free of bounds.
You didn't italize or mark the phrase, but I see one good candidate:
"The circus animal trainer" is in a way another name given to Mervin, a kind of "renaming" him: this is called an appositive phrase, so if this was the phrase appositive phrase is the answer! (also, I don't see the other phrases here).
first one is (MAIN CLAUSE)
second one is (SUBORDINATING CLAUSE)
but i am not sure about the 3rd one
