The most common and most likely answer to this crossword clue is the 7 letter word PUFFINS
I've seen this crossword clue in The Guardian
P
U
F
F
I
N
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 = puffins.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 PUFFINS mean?
PUFFINS noun- The young of the Manx shearwater (Puffinus puffinus), especially eaten as food.
- The Atlantic puffin (Fratercula arctica) or (by extension) any of the other various small seabirds of the genera Fratercula and Lunda that are black and white with a brightly-coloured beak.
- Any of various African and Asian pierid butterflies of the genus Appias. Some species of this genus are also known as albatrosses.
- A puffball.