Ibn Ezra on Leviticus 11:29

וזה לכם הטמא. במגע גם אלה השמנה לא נוכל לדעתם באמת גם העופות כי אם מדברי קבלה: AND THESE ARE THEY WHICH ARE UNCLEAN UNTO YOU. By touch.⁷⁷One becomes unclean by touching their dead carcasses. We also are not able to actually know what these eight creatures⁷⁸Mentioned in verses 29 and 30. refer to. This is also true of the fowl.⁷⁹Mentioned in verses 13-19. We can only know their identity by tradition.