A birth certificate doesn't prove citizenship. Someone born in the US could emigrate and become a citizen of another country, then re-enter the US on a visit. C

A birth certificate doesn't prove citizenship. Someone born in the US could emigrate and become a citizen of another country, then re-enter the US on a visit. Conversely, someone born elsewhere could be a naturalized citizen. A passport could prove citizenship (assuming the passport office checks properly before issuing it), but not everyone has a passport.