water hen vs gallinule

water hen and gallinule both are nouns.

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

As nouns, water hen and gallinule are synonyms defined as:

water hen and gallinule: any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens

Other synonyms of water hen include marsh hen, swamphen.

water hen (noun)	gallinule (noun)
any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens	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