A person may have acquired United States citizenship by being born in the United States, Puerto Rico, the U.S. Virgin Islands, or Guam.[43]  The INA also allows “a person of unknown parentage found in the United States while under the age of five years” to be considered a citizen of the United States unless the person is “shown, prior to his attaining the age of twenty-one years, not to have been born in the United States.”[44]

[43] See INA § § 301(a) & (b), 302, 304-307, 8 U.S.C. § § 1401(a) & (b), 1402, 1404-1407 (citizen by birth in the United States, Puerto Rico, U.S. Virgin Islands, or Guam).

[44] INA § 301(f), 8 U.S.C. § 1401(f).

Second Circuit

CITIZENSHIP - ACQUIRED CITIZENSHIP - BIOLOGICAL PARENTAGE IS NECESSARY TO A CLAIM TO UNITED STATES CITIZENSHIP UNDER FORMER INA 301(a)(3)
Colaianni v. INS, 490 F.3d 185 (2d Cir. Jun. 15, 2007) (per curiam) (petition for review denied on basis that noncitizen's claim to be United States citizen, advanced as a defense in removal proceedings, was invalid, since biological parentage is necessary for a person to claim citizenship under former INA 301(a)(3) because of the plain language of the statute, which refers to persons "born ... of parents both of whom are citizens of the United States" and pertains only to the acquisition of citizenship "at birth."), citing Marquez-Marquez v. Gonzales, 455 F.3d 548, 556-57 (5th Cir.2006) (rejecting the same argument based upon a plain reading of the statute).

Fifth Circuit

CITIZENSHIP " BIRTH ON U.S. MILITARY BASE ABROAD DOES NOT CONFER U.S. BIRTHRIGHT CITIZENSHIP
Thomas v. Lynch, ___ F.3d ___, 2015 WL 4745688 (5th Cir. Aug. 7, 2015) (petitioner's birth on military base did not render him a birthright citizen under the Fourteenth Amendment).

CITIZENSHIP - BIRTH IN UNITED STATES
Lopez v. Holder, ___ F.3d ___, 2009 WL 682991 (5th Cir. Mar. 17, 2009) (without consent of petitioners, magistrate judge did not have jurisdiction to try citizenship case based on birth).

Other

CITIZENSHIP - PROOF OF - CENSUS INFORMATION
United States Citizenship may be sometimes proven by showing a child appeared on a U.S. census in the United States shortly after birth. Contact US Census Bureau. Thanks to Mary Lee Eldridge