RTF is a global modification that replaces all towns in the game.

Not quite... I understand that mages summon spirits or something similar as various kinds of birds (correct me if I'm wrong). This means that, among other things, the mage influences the material form that the summoned spirit takes.

They aren't told what form to take. Quote from the description: "Powerful spirits often take the form of majestic birds. Apparently, that's why the strongest entities that mages managed to summon in the House of Anima took the form of ghostly eagles, woven from magic." The spirits themselves took the form of birds, and the mages summoned them. But if I'm wrong, please correct me.

Added 5 minutes later
I changed the color to match the green dragons. Is it better?