Mountain bird briefly returns (4)

I believe the answer is:
berg
'mountain' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for berg that I've seen before include "He composed" , "trouble at sea" , "Alban ----, composer of the incomplete opera 'Lulu' (1935)" , "Lump of ice" , "One who scored" .)
