marsh wren vs wren

marsh wren and wren both are nouns.

Word	Noun	Adjective	Verb	Adverb
marsh wren	Yes	No	No	No
wren	Yes	No	No	No

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

marsh wren: a wren of the genus Cistothorus that frequents marshes
wren: any of several small active brown birds of the Northern Hemisphere with short upright tails; they feed on insects

Other hypernyms of marsh wren include jenny wren.

marsh wren (noun)	wren (noun)
a wren of the genus Cistothorus that frequents marshes	any of several small active brown birds of the Northern Hemisphere with short upright tails; they feed on insects