herring gull vs sea gull

herring gull and sea gull both are nouns.

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

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

herring gull: large gull of the Northern Hemisphere
sea gull: mostly white aquatic bird having long pointed wings and short legs

Other hypernyms of herring gull include gull, seagull.

herring gull (noun)	sea gull (noun)
large gull of the Northern Hemisphere	mostly white aquatic bird having long pointed wings and short legs