The siathi have wings. An average wing span for a full adult is approximately twelve feet. The main problem with their anatomy is that their bones have more in common with a human’s skeleton rather than a bird’s. While they are still significantly lighter than a human of the same size, their bones are still not quite light enough to allow for flight.
In order to achieve flight, they have a special kind of feather spread out in the wings themselves. These feathers are saturated with magic. Without them, the wings would merely be very heavy decorations and would only allow the siathi to glide on wind currents.
There is an old legend that if you were to gather enough of these feathers that a species that wouldn’t otherwise be capable of flight would then be able to. This has never been proven or disproved. These particular feathers take a very long time to grow and only molt after several years and even then only one at a time from each wing. Tests have shown that the molted feathers retain little of their innate magic. It was learned by accident that if you were to physically pull the feathers from the wings, they retain all of their magic.