The most common and most likely answer to this crossword clue is the 3 letter word ANI
I've seen this crossword clue in The Times
A
N
I
Buddy explains!

The straight part
We know that "tropical bird" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tropical bird = ani.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ANI mean?
ANI noun- A bird of the genus Crotophaga in the cuckoo family.
- The lower orifice of the alimentary canal, through which feces and flatus are ejected.