The most common and most likely answer to this crossword clue is the 4 letter word LOON
L
O
O
N
Buddy explains!

The straight part
We know that "bird" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bird = loon.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 LOON mean?
LOON noun- An idler, a lout.
- A boy, a lad.
- A harlot; mistress.
- A simpleton.
- A crazy or deranged person.
- An English soldier of an expeditionary army in Ireland.
- Any of various birds, of the order Gaviiformes, of North America and Europe that dive for fish and have a short tail, webbed feet and a yodeling cry.