When Billie Eilish chose “Happier Than Ever” as the title of her previous album, you didn’t have to look far past the straight-faced album cover to find levels of irony in that. She didn’t completely put on a happy face for this year’s full-length follow-up, “Hit Me Hard and Soft,” either. But one track, “Birds of a Feather,” did find Eilish basking in something very close to sunlight, with its pretty, percolating feel and nearly out-of-character romantic optimism. When the ”Hit Me” album first landed, fans drove three tracks into the upper ranks of the Billboard Hot 100 (also including “Lunch” and “Chihiro”). But it’s the feather-light touch of “Bird” that stuck around longest and became her biggest hit to date, as well as landing Grammy nods for record and song of the year and pop solo performance.

Says Finneas, her co-writer, producer and brother: “In some ways. I’m sort of extra-mistrustful when people tell me that they think something’s gonna be a big hit, because it feels like jinxing it — or because, usually, people say that about stuff that sounds like something else that already exists. You know, nobody said that about ‘Bad Guy’ before it came out, and I think the reason nobody said that was that it really sounded new.  Anyway, that being said, every time we played “Birds of a Feather” for the label or our close friends, they would kind of freak out.”

Which didn’t mean there was any rush to jump on it, and if anything, “Lunch” was the first unofficial emphasis track. “I think one of the reasons we didn’t want to put a single out — and the reason we didn’t immediately lean into ‘Birds’ —  is that we are careful about how singles make you perceive something differently,” Finneas says. “If we’d put out ‘Birds of a Feather’ as a single before the album, people are gonna go, ‘Wow, this is a real pivot. Is everything on the album gonna sound like this?’ And the truth was, it’s an outlier. So the fact that ‘Birds of a Feather’ has gone on to have this insane life is such a thrill to both of us. We loved making it, but it’s extra-validating to have something blow up like that without pushing it as a single.” That is, the song took flight before it did officially become the album’s third radio track in July, scaling the top of the pop, adult pop and rock/alternative airplay charts.

Wait, did he say they “loved” making it? Finneas seems to be of two minds about that. It took 11 months to make, and “to be honest, that is excruciating,” he admits, “because you keep second-guessing it. As we delved deeper, we became aware that to make the whole production feel finished, but as good as the demo we loved, was gonna be this real challenge. Billie was driven crazy by her vocal performance and redid that a bunch of times, and I added all these layers of production and then would take ’em out and add ’em again. I have so many memories of listening to version 5 of the song, driving around L.A. and just being like, ‘I’ve lost my touch. I’m so bad right now.’ I think it speaks to how alien the song is to the rest of our catalog, that a lot of what I’d learned from the rest of the music I’ve made with her didn’t translate. I had to really kind of reinvent the wheel on this song.”

Andrew Marshall, Eilish’s career-long touring drummer, makes one of his first recorded appearances on the track, but he doesn’t completely crowd out the electronics. “One of the things we’ve moved towards as Billie’s career has gone on is trying to make music that feels like it might be a little bit irrelevant what year it is,” Finneas says. “Some of the things that age most quickly are electronic drums, which I’ve used tons of, but they usually sound very of their time. So the drums on this song — real and programmed — all have a kind of organic feel, which helps them sound timeless. There’s a lot of acoustic guitar, which I think has never really gone in or out of style. And even the synths have a kind of a vintage quality that isn’t as bleeding-edge as some pop music going on right now.” The end result is a little bit bubbly R&B, a little bit finger-picky Glen Campbell and a whole lot of pure Billie Eilish on a lovestruck bender.