The Great Bird of the Galaxy was a mythological creature evoked as a blessing. In 2266, after Janice Rand brought him some food, Hikaru Sulu said "May the Great Bird of the Galaxy bless your planet!" (TOS: "The Man Trap")
Background
The Great Bird
The Great Bird of the Galaxy was the nickname of Star Trek creator Gene Roddenberry. It was used in the original series as an inside joke. The "Great Bird of the Galaxy", which, according to Peter David, is actually a giant bird, appears in the Star Trek: New Frontier novels.
An early Okudagram was later created during TNG Season 1 that depicted an image of the Great Bird – Gene's face placed on the body of a parrot wearing a Starfleet uniform.