marsh wren vs jenny wren

marsh wren and jenny wren both are nouns.

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

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

marsh wren: a wren of the genus Cistothorus that frequents marshes
jenny 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 wren.

marsh wren (noun)	jenny 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