2016 May 20ᵗʰ. Feathers Across the Seasons!

おはよう!

So. Songs. VOCALOID.

REMEMBER ME?!

今日

First of all, you have to listen to the song. Got it now? Good. So,
This song, if you noticed it in the title or remembered the characters, is sung by Kagamine Rin and Len, two vocaloids who usually are implied to be siblings but in this case are portrayed as a couple. That is neat. But secondly,
WHAT THE HECK?!
I mean, I understand the basic story. But still, I don't get the part about the feathers. Someone, enlighten me! Please!
I require aid ;-;