The most common and most likely answer to this crossword clue is the 11 letter word CHAFFINCHES
I've seen this crossword clue in Financial Times
C
H
A
F
F
I
N
C
H
E
S
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 = chaffinches.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 CHAFFINCHES mean?
CHAFFINCHES noun- A small passerine bird, Fringilla coelebs, of the finch family.