marsh hen vs aquatic bird

marsh hen and aquatic bird both are nouns.

Word	Noun	Adjective	Verb	Adverb
marsh hen	Yes	No	No	No
aquatic bird	Yes	No	No	No

As nouns, aquatic bird is a hypernym of marsh hen; that is, aquatic bird is a word with a broader meaning than marsh hen:

marsh hen: any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens
aquatic bird: wading and swimming and diving birds of either fresh or salt water

marsh hen (noun)	aquatic bird (noun)
any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens	wading and swimming and diving birds of either fresh or salt water
a coot found in North America