[FreeReign]

There’s nothing on his birth certificate that says his mother was U.S. citizen at the time of his birth.

How is that different from any birth certificate?

[Behind the Blue Wall]

It’s not. A birth certificate is only good to prove citizenship for the nation in which it was issued. A U.S. birth certificate is sufficient to prove U.S. citizenship; a Canadian birth certificate is sufficient to prove Canadian citizenship. A Canadian birth certificate is not sufficient to prove U.S. citizenship, and a U.S. birth certificate would not be sufficient to prove Canadian citizenship. That’s why as it relates to U.S. citizenship, there’s a document called the Consular Report of Birth Abroad of a Citizen of the United States of America (CRBA).

[Mariner]

"How is that different from any birth certificate? "

It's not, but the paper trail is officially recorded and traceable. It then becomes possible that the parents status can be determined.

It's how those ancestry sites work, they are gradually converting all birth and death records...official records of many types into digital information.