songster vs songstress

songster and songstress both are nouns.

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

As nouns, songstress is a hyponym of songster; that is, songstress is a word with a more specific, narrower meaning than songster:

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