purple gallinule vs marsh hen

purple gallinule and marsh hen both are nouns.

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

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

purple gallinule: gallinules with showy purplish plumage
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

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

purple gallinule (noun)	marsh 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