mew gull vs sea gull

mew gull and sea gull both are nouns.

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

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

mew gull: the common gull of Eurasia and northeastern North America
sea gull: mostly white aquatic bird having long pointed wings and short legs

Other hypernyms of mew gull include gull, seagull.

mew gull (noun)	sea gull (noun)
the common gull of Eurasia and northeastern North America	mostly white aquatic bird having long pointed wings and short legs