Flying Creatures as mediocre as they are common.
They have Air Mount for traversing the overworld.
They have no value at your Farm as they don't produce anything.
However as one of the few units in-game that have no skills whatsoever, they make a good template for messing around, especially with the additional options that some Essences give. If you think you've learned all there is to learn about how the Farm works, try making yourself a custom Bird. For example, make a "thunderbird" using both breeding and fusing. It should have Magic or Dexterity Level Type, at least one electrical spell, and it should be bigger than its parents.
They can be sold for 100 gold at Medium size and for 300 gold at Enormous size. Every level above 1 additionally increases their price by 5%.
They are offered by Seliel during her events.
Futa Potions have no effect on them. Not that you'd want to waste one on a bird anyway.