pewit gull vs sea gull

pewit gull and sea gull both are nouns.

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

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

pewit gull: small black-headed European gull
sea gull: mostly white aquatic bird having long pointed wings and short legs

Other hypernyms of pewit gull include gull, seagull.

pewit gull (noun)	sea gull (noun)
small black-headed European gull	mostly white aquatic bird having long pointed wings and short legs