Start an expedition to Ancient Forest or Wildspire Waste (camp and daytime does not matter) and check in both areas the non-hostile creatures like Aptonoth and Apceros with your equipped Ghillie Mantle (Important!). Atop of these creatures rides our creature, a bird.
These Birds are a ultra rare spawn! The spawn can anytimes happens and are the birds there you hear a sound! Don't get too close to the birds without the Ghillie Mantle, otherwise they flee!