aquatic bird vs seafowl

aquatic bird and seafowl both are nouns.

Word	Noun	Adjective	Verb	Adverb
aquatic bird	Yes	No	No	No
seafowl	Yes	No	No	No

As nouns, seafowl is a hyponym of aquatic bird; that is, seafowl is a word with a more specific, narrower meaning than aquatic bird:

aquatic bird: wading and swimming and diving birds of either fresh or salt water
seafowl: a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.

aquatic bird (noun)	seafowl (noun)
wading and swimming and diving birds of either fresh or salt water	a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.