What evidence might that be?
Both my parents were born here but I have no proof i could or would show some bureaucrat.
Birth Certificates. I have them for my late parents, Born 1920 & 1927, and also for my also deceased maternal grandparents, born 1901 & 1911, as they were needed for identification for local, state & federal this & that years and years ago.
Since my father's parents died in the Great Depression (1930 & 1936) 11 plus years before I was born, I did not ever need them but if necessary, I would have found them.
I could show family pictures of father, grandfather, and great grandfather here in the US. But you’re right about birth certificates. How many actually easily have acess to their parents’ birth certificates?