From the Super Mario Wiki, the Mario encyclopedia
The Black Jibberjay is a character that appears only in Super Mario Galaxy 2. He is the leader of the Jibberjays and the fastest of them all. His first appearance is in the Wild Glide Galaxy where, along with his Jibberjay followers, sends a letter challenging Mario and Fluzzard to a race to prove who is the fastest. Mario must ride Fluzzard through the trees and get to the finish line faster than Black Jibberjay and all the other Jibberjays to win the race. After Mario and Fluzzard win the race, the Black Jibberjay is so impressed that he gives Mario a Power Star as a trophy and declares him the new leader of the Jibberjays.
The Black Jibberjay later sends another letter to Mario, requesting that Mario races him again, this time in the Fleet Glide Galaxy. Mario hops on Fluzzard and races the Black Jibberjay again, but this time he races Mario alone, without his Jibberjay followers. After Mario beats him once again, the Black Jibberjay gives Mario another Power Star and does not appear again.