The person I responded to thinks a birth certificate is sufficient to prove citizenship. My birth certificate can not be used to establish citizenship even though it was issued by the U.S. government and I’m a natural born U.S. citizen.

It works the other way, too; I know a guy whose parents were US diplomatic personnel stationed in Tehran at the time of his birth. His US passport says he's from Iran, which is technically true, though he's not Iranian and he's never held Iranian citizenship.

In such cases, there is usually a “consular report of a birth abroad” that the embassy sends to Washington and the parents get a copy of. Military servicemembers also frequently get this. It’s not required to be done by any means and it is not a birth certificate though it can usually be used in place of one.

But! I think it may be the only document issued about a birth that is proof of US citizenship. And, paradoxically, is not available to people born inside the US.

I got my certificate of citizenship a few days after birth. I believe anyone can get this but almost no one has heard of this.