ivory gull vs seagull

ivory gull and seagull both are nouns.

Word	Noun	Adjective	Verb	Adverb
ivory gull	Yes	No	No	No
seagull	Yes	No	No	No

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

ivory gull: white Arctic gull; migrates as far south as England and New Brunswick
seagull: mostly white aquatic bird having long pointed wings and short legs

Other hypernyms of ivory gull include gull, sea gull.

ivory gull (noun)	seagull (noun)
white Arctic gull; migrates as far south as England and New Brunswick	mostly white aquatic bird having long pointed wings and short legs