From the Super Mario Wiki
Pidgit Bills are Pidgits that take the place of Bullet Bills in Super Mario World after Mario or Luigi clear the Special World in the game (or clears all 96 exits in the remake).
Instead of flying on carpets like the Pidgits from Super Mario Bros. 2, Pidgit Bills fly in the same manner that Bullet Bills do. Other than appearance, there is no real difference between Bullet Bills and Pidgit Bills. In fact, in the original game, Pidgit Bills don't appear to even flap their wings in order to fly, although they are depicted with flapping wings in the Game Boy Advance remake. Similarly, the original version of Super Mario World does not have Pidgit Bills going diagonally or vertically, and instead continues to use Bullet Bill sprites, but this was fixed in the Game Boy Advance remake.