Birds of a Feather

Eilish wrote it with Finneas, who recorded and produced it at his home studio in Los Angeles. A teaser trailer for the third season of Heartstopper gave the first preview on 13 May 2024, and the song came out with the album on 17 May. It became the second official single on 18 June. The title borrows the idiom "birds of a feather flock together". It is in D major at 105 beats per minute. She told Apple Music it holds the highest belt of her career.

It debuted at 13 on the Billboard Hot 100 and later peaked at two, topping the Billboard Global 200 and spending 52 weeks on the Hot 100, more than Bad Guy's 49. It also reached number two in the UK. It was Spotify's most-streamed song in a calendar year in 2024 and is the most-streamed solo song by a woman on the service. The IFPI ranked it the fourth best-selling song worldwide that year. Aidan Zamiri directed the video, released on 27 September 2024 and filmed at the Westin Bonaventure Hotel in Los Angeles.

Alexis Petridis of the Guardian singled out the line in which she pledges love until "I rot away, dead and buried". It was nominated for Song of the Year, Record of the Year and Best Pop Solo Performance at the 2025 Grammys.