x1 = d1 = c1 is a member of the class of birds which walk/run/hop as their characteristic/primary form of locomotion, of species/taxon x2 = c2
x2 should be enough to imply the environment in which these creatures live (and thus on what surfaces they walk) as well as the limbs with which they walk. These birds should essentially be flightless. Many ratites, penguins, and extinct species qualify; additionally, in some contexts, domesticated chickens and ducks might qualify (at least as ta'e dzucpi). Walking must be a fundamental characteristic of the bird. The class of such birds does not form a phylogenic tree or taxon; it is just a conceptual organizer/bin to be used for identifying birds with similar, fundamentally important, physical characteristics. The term "flightless bird" may be used, but the term is basically synonymous with his word only by circumstance/happenstance, not by logic: it is conceivable that, in another universe, there could exist a bird which is flightless but which does not primarily/characteristically walk; thus, flightless birds form a superclass which contains (modulo some fuzziness) dzucpi, but in reality, the relative complement of these classes is empty. See also: voltolka'ecpi