The most common and most likely answer to this crossword clue is the 5 plus 7 letter word BRUSH TURKEYS
I've seen this crossword clue in The Guardian
B
R
U
S
H
T
U
R
K
E
Y
S
Buddy explains!
The straight part
We know that "Australian birds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case australian birds = brush turkeys.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 BRUSH TURKEYS mean?
BRUSH TURKEYS noun- Any of several species of large birds, found in Australia and New Guinea, in the megapode (turkey-like) family Megapodiidae.