purple gallinule vs water hen

purple gallinule and water hen both are nouns.

Word	Noun	Adjective	Verb	Adverb
purple gallinule	Yes	No	No	No
water hen	Yes	No	No	No

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

purple gallinule: gallinules with showy purplish plumage
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

Other hypernyms of purple gallinule include gallinule, marsh hen, swamphen.

purple gallinule (noun)	water hen (noun)
gallinules with showy purplish plumage	any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens
	a coot found in North America