cob vs sea gull

cob and sea gull both are nouns.

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

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

cob: white gull having a black back and wings
sea gull: mostly white aquatic bird having long pointed wings and short legs

Other hypernyms of cob include gull, seagull.

cob (noun)	sea gull (noun)
adult male swan	mostly white aquatic bird having long pointed wings and short legs
stocky short-legged harness horse
white gull having a black back and wings
nut of any of several trees of the genus Corylus