When I see comments along the lines of "why does the owl have shackles", "why is it tethered", and other similar questions, I sincerely wonder: what and where are you looking at?
The birds have leather cuffs with straps on their feet. They are soft and do not even harm the feathers under them. The function of these cuffs is the same as a dog's collar.
Yoll has a metal aluminium ring permanently attached to her leg. It cannot be removed without damaging it. It's her passport and proof of origin. But the ring is not used for tethering the bird - it just hangs on the leg with no impact on the bird's health. An address tag with my phone number is attached to the ring, just in case.
Also, birds can tear the leather with their beaks if they wish to do so. But they don't do this, because they perceive cuffs with straps as their feathers and therefore as part of their body. When the birds are indoors, their movement is not constrained, as they're not tethered.
Moreover, jesses are necessary for a tame bird, for taming a bird, and for convenience and proper interaction with a bird.