moorhen vs marsh hen

moorhen and marsh hen both are nouns.

Word	Noun	Adjective	Verb	Adverb
moorhen	Yes	No	No	No
marsh hen	Yes	No	No	No

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

moorhen: black gallinule that inhabits ponds and lakes
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 moorhen include gallinule, swamphen, water hen.

moorhen (noun)	marsh hen (noun)
black gallinule that inhabits ponds and lakes	any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens
female red grouse	a coot found in North America