sea gull vs blackcap

sea gull and blackcap both are nouns.

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

As nouns, blackcap is a hyponym of sea gull; that is, blackcap is a word with a more specific, narrower meaning than sea gull:

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

sea gull (noun)	blackcap (noun)
mostly white aquatic bird having long pointed wings and short legs	small brownish-grey warbler with a black crown
	chickadee having a dark crown
	small black-headed European gull
	raspberry native to eastern North America having black thimble-shaped fruit