kittiwake vs sea gull

kittiwake and sea gull both are nouns.

Word	Noun	Adjective	Verb	Adverb
kittiwake	Yes	No	No	No
sea gull	Yes	No	No	No

As nouns, sea gull is a hypernym of kittiwake; that is, sea gull is a word with a broader meaning than kittiwake:

kittiwake: small pearl-grey gull of northern regions; nests on cliffs and has a rudimentary hind toe
sea gull: mostly white aquatic bird having long pointed wings and short legs

Other hypernyms of kittiwake include gull, seagull.

kittiwake (noun)	sea gull (noun)
small pearl-grey gull of northern regions; nests on cliffs and has a rudimentary hind toe	mostly white aquatic bird having long pointed wings and short legs