So it seems that “mesorah” in the case of birds does NOT mean from Sinai, but means only “a well established custom among the frum Jews”, which leaves open the question of how such a “mesorah” comes into being.
There is no Mesora from Sinai on all Kosher birds. There is only a Mesora on which are the NON kosher birds listed in the Torah. Being that today we no longer know which birds are non Kosher we, therefore, need a mesora on which birds are kosher. Once we know they were always eaten we also know that they aren’t from the non kosher birds the Torah lists