Gull-like bird (4)
(Other definitions for tern that I've seen before include "Rent out the bird" , "Fork-tailed seabird" , "Gull-like sea bird" , "Gull-like seabird" , "Three-masted schooner" .)
(Other definitions for skua that I've seen before include "Large predatory seabird" , "Large sea bird" , "Flyer" , "Seabird; sounds like pin" , "Craziness" .)