choth uses a gravbelt instead of wings and goggles instead of a nictitating membrane.
There are other differences. Ythrians are intelligent hunting carnivores who eat live prey. They are biologically territorial. All choth members are subject to laws and customs that allow deathpride quarrels to be resolved by duels, although Kruaths are peace-holy.
Avalonian choths and Kruaths coexist with the Parliament of Man but human beings who "go bird" by joining a choth are under choth, not human, law. Some Ythrians become "walkers" by adopting the human custom of living as atomic individuals within the planetary community.
Some Star Trek fans study Vulcanism. Poul Anderson fans need to understand Ythrianism, which includes an Old and a New Faith.