Rue's Four Note Song

When Rue of District 11 worked high up in the trees, when she saw the quitting time flag, she would use this song to tell her fellow harvesters when the working day was over. She would sing the tune into the trees and the surrounding mockingjays would pick up the song and sing it back to those on the ground.

The Hunger Games
During the 74th Hunger Games, Rue teams up with Katniss and whistles the song to let Katniss know she's safe and okay. Rue shows Katniss her tune while Katniss chooses a different tune, whose notes are G, B♭, A, D. Katniss then leaves to destroy the Careers' camp, but upon her return, she hears Rue scream. When Katniss reaches Rue, tangled in a net, she saw Marvel from District 1 send a spear through Rue's abdomen, ultimately killing her.​ Katniss kills Marvel and rushes to Rue, who asks that Katniss sing her a song. Katniss obliges and, once Rue has passed, covers her in wildflowers. As she walks away, she sings Rue's little tune and has many dreams thereafter of Rue singing the song. In one dream, Katniss is being taught the song by Rue herself.

Catching Fire
During the Victory Tour an old man whistles Rue's song to honor her. After the event, he was shot in the back of the head on the steps of the district building, along with two other people, in front of the crowd. This was also a prearranged signal for District 11 to perform District 12's honorary salute.

Trivia

 * At the end of every The Hunger Games trailer (with the exception of the first sneak peek), four whistled notes can be heard. These four notes, being Katniss' four-note whistle, are the same first four notes of the love theme from the 1968 film adaptation of Romeo & Juliet. Interestingly enough, the song, entitled What Is A Youth?, starts with that very question and those same four whistled notes might be posing that question to The Capitol.