songstress vs songster

songstress and songster both are nouns.

Word	Noun	Adjective	Verb	Adverb
songstress	Yes	No	No	No
songster	Yes	No	No	No

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

Other hypernyms of songstress include adult female, woman.

songstress (noun)	songster (noun)
a woman songster (especially of popular songs)	any bird having a musical call
	a person who sings with skill
	a composer of words or music for popular songs