>It seems unlikely to me that Zild is the only place in the world to call them tortoiseshell.

from W3:
tortoiseshell (cat): a usually female domestic cat having a black, red, and cream or white blotched coat often with a white blaze on the face

calico: 3 : a blotched or spotted animal:

so, according to the prime USn dictionary, tortoiseshell is more specifically a cat than is calico.