water hen vs aquatic bird

water hen and aquatic bird both are nouns.

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

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

water 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

water 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