The most common and most likely answer to this crossword clue is the 7 letter word CHIRRUP
I've seen this crossword clue in The Times
C
H
I
R
R
U
P
More clues leading to the result CHIRRUP
- Cross cat’s sound recalled sound of bird
- Tweet greeting bishop in competition
- . Church support overlooking holy man’s tweet
- Tweet, greeting runs in competition
- Pointless letter in academic press to Twitter
- Twitter, initially, could help idiots sound happy when upset
- Greek character raised contented noise to sound cheerful
- Tweet letter: pleased Tom might do this in retirement
Buddy explains!

The straight part
We know that "sound of bird" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sound of bird = chirrup.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 CHIRRUP mean?
CHIRRUP noun- A series of chirps, clicks or clucks.
- To make a series of chirps, clicks or clucks.
- To express by chirping.
- To quicken or animate by chirping.