bird-on-the-wing vs milkwort

bird-on-the-wing and milkwort both are nouns.

Word	Noun	Adjective	Verb	Adverb
bird-on-the-wing	Yes	No	No	No
milkwort	Yes	No	No	No

As nouns, milkwort is a hypernym of bird-on-the-wing; that is, milkwort is a word with a broader meaning than bird-on-the-wing:

bird-on-the-wing: common trailing perennial milkwort of eastern North America having leaves like wintergreen and usually rosy-purple flowers with winged sepals
milkwort: any of various plants of the genus Polygala

bird-on-the-wing (noun)	milkwort (noun)
common trailing perennial milkwort of eastern North America having leaves like wintergreen and usually rosy-purple flowers with winged sepals	any of various plants of the genus Polygala