Migratory bird (6)
(Other definitions for cuckoo that I've seen before include "Bird (with a bad reputation)" , "building no nest?" , "flier?" , "counting as bird? Yes and no" , "Bats" .)
'migratory bird' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tern that I've seen before include "Winger" , "Pay rent for seabird" , "Fork-tailed relative of the gull" , "Fork-tailed seabird" , "Three-masted schooner" .)