Bird or birds migrating west (4)
I believe the answer is:
skua
'bird' is the definition.
(skua is a kind of bird)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for skua that I've seen before include "Predatory northern seabird" , "Large predatory seabird" , "Flyer" , "Craziness" , "Brownish sea bird" .)