Birdy was a homemade robot, originally built by Athrun Zala, who gave it to Kira Yamato in C.E. 68 as a parting gift when he had to return to PLANT. Kira treasured Birdy as a reminder of Athrun, and often carried him on his shoulder. Birdy remained with Kira throughout both the First and Second Alliance-PLANT War.

(Source: The Gundam Wiki)