One bird or more soaring? (4)

I believe the answer is:

skua

I'm a little stuck... Click here to teach me more about this clue!

'one bird' is the definition.
(skua is a kind of bird)

I cannot understand how the rest of the clue works.

(Other definitions for skua that I've seen before include "Large sea bird" , "Brownish sea bird" , "Predatory northern seabird" , "Gull-like rapacious bird" , "Flyer" .)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app