The most common and most likely answer to this crossword clue is the 4 letter word HENS
I've seen this crossword clue in The Guardian
H
E
N
S
More clues leading to the result HENS
Buddy explains!

The straight part
We know that "birds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case birds = hens.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 HENS mean?
HENS noun- A female chicken (Gallus gallus), particularly a sexually mature one kept for its eggs.
- A female of other bird species, particularly a sexually mature female fowl.
- A female fish (especially a salmon or trout) or crustacean.
- A woman, particularly
- A hen night.
- An affectionate term of address used to women or girls.
- A henlike person of either sex.
- The hard clam (Mercenaria mercenaria), a bivalve shellfish.