Birthday ~Aruki Hajimeta hi~ (Birthday ~歩き始めた日~, Birthday ~A Day that Started to Walk~) is the 1st track to Aozu's mini-album Aozu no Singles Best. It is also track 11 from Fuji's album Serendipity and track 5 from Tezuka's album Impressive. There is also a version where Aozu and Cap to Bin sing as a whole, released as a single.