The White Doves released from the castle of Mount Ji City were not real pigeons but enslaved beasts crafted by Zero. They could fly at high speeds and even sustain themselves by preying on smaller creatures. Their purpose was simple, much like that of carrier pigeons: they were used to deliver messages.
However, Zero had devised a unique method for these avian enslaved beasts to deliver messages. They could generate a colored gas within their bodies and then expel it to form words in the sky. The words they created were clear, much like the skywriting of an aerobatic display team, and they lingered for a long time without dispersing.
Real birds don't have symbiotic microbial colonies in their digestive tracts and naturally don't pass gas, but these pigeon enslaved beasts were not real birds.