To be a Natural Born Subject or Citizen of a particular country means to be born into and under the full allegiance of a particular country. It is distinct from a citizen who may obtain the rights of citizenship by right of statute. Ted was, without any doubt, born into the allegiance of Canada and, by their own laws, he is considered a Natural Born Citizen of Canada.
Petitioner, having been born outside the territory of the United States, is an alien as far as the Constitution is concerned, and "can only become a citizen by being naturalized, either by treaty, as in the case of the annexation of foreign territory; or by authority of Congress."
Dung Van Chau, Petitioner, v. Immigration and Naturalization Service, Respondent, 247 F.3d 1026, (9th Cir. 2001)
However, evidence of foreign birth gives rise to a rebuttable presumption of alienage, shifting the burden to the respondent or deportee to prove citizenship.