The most common and most likely answer to this crossword clue is the 14 letter word ORNITHOLOGICAL
I've seen this crossword clue in The Times
O
R
N
I
T
H
O
L
O
G
I
C
A
L
Buddy explains!
The straight part
We know that "of birds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of birds = ornithological.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.