The most common and most likely answer to this crossword clue is the 7 letter word KAKAPOS
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Buddy explains!

The straight part
We know that "Nocturnal birds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nocturnal birds = kakapos.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 KAKAPOS mean?
KAKAPOS noun- A large flightless parrot, Strigops habroptilus, with greenish plumage, that is nocturnal and native to New Zealand.