This song is one of the most complicated Vocaloid duets to sing, for it requires at least two people, with different lyrics and melodies, which makes the song more unique. The single album "Yumemiru Kotori" was released on KarenT to celebrate the Kagamine Rin/Len Append's first anniversary. Also, in June 2009, the song was available on JOYSOUND, and it is featured in the EXIT TUNES compilation album Vocalostar.
The lyrics of the song use a number of metaphors, so there are many different interpretations. One of the most common is that two friends/lovers are trying to face a problem/catastrophe. Although they try to help each other, both of them have changed greatly, in comparison to the wonderful past they had before. In short, it's a song of desperate hope, love, and freedom symbolized by two birds in a cage.
NOTE: Lyrics marked as both do not always mean they are singing together, rather the same lyrics at different times. Vice versa for the different colored lyrics; they may be singing at the same time, despite being marked separately.
Also, due to the complexity of the song's timing of the lyrics, some of these may be wrong. Feel free to correct what you think is wrong.