Billie Eilish is leaning in to the soft side of her third studio album, Hit Me Hard and Soft. On Monday, the singer and songwriter released a series of live performances stripping four songs from the record down to their acoustic forms. The selections — including “Birds of a Feather,” “Skinny,” “Wildflower,” and “L’Amour de Ma Vie” — are now available as part of Songline, the first installment of Amazon Music’s new songwriting series.

For the performances, Eilish — joined by Finneas — was backed by a robust string section and a live...