songstress vs adult female

songstress and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
songstress	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of songstress; that is, adult female is a word with a broader meaning than songstress:

Other hypernyms of songstress include songster, woman.

songstress (noun)	adult female (noun)
a woman songster (especially of popular songs)	an adult female person (as opposed to a man)